gtfs-realtime.proto

// Copyright 2015 The GTFS Specifications Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Protocol definition file for GTFS Realtime.
//
// GTFS Realtime lets transit agencies provide consumers with realtime
// information about disruptions to their service (stations closed, lines not
// operating, important delays etc), location of their vehicles and expected
// arrival times.
//
// This protocol is published at:
// https://github.com/google/transit/tree/master/gtfs-realtime

syntax = "proto2";
option java_package = "com.google.transit.realtime";
package transit_realtime;

// The contents of a feed message.
// A feed is a continuous stream of feed messages. Each message in the stream is
// obtained as a response to an appropriate HTTP GET request.
// A realtime feed is always defined with relation to an existing GTFS feed.
// All the entity ids are resolved with respect to the GTFS feed.
// Note that "required" and "optional" as stated in this file refer to Protocol
// Buffer cardinality, not semantic cardinality.  See reference.md at
// https://github.com/google/transit/tree/master/gtfs-realtime for field
// semantic cardinality.
message FeedMessage {
  // Metadata about this feed and feed message.
  required FeedHeader header = 1;

  // Contents of the feed.
  repeated FeedEntity entity = 2;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// Metadata about a feed, included in feed messages.
message FeedHeader {
  // Version of the feed specification.
  // The current version is 2.0.  Valid versions are "2.0", "1.0".
  required string gtfs_realtime_version = 1;

  // Determines whether the current fetch is incremental.  Currently,
  // DIFFERENTIAL mode is unsupported and behavior is unspecified for feeds
  // that use this mode.  There are discussions on the GTFS Realtime mailing
  // list around fully specifying the behavior of DIFFERENTIAL mode and the
  // documentation will be updated when those discussions are finalized.
  enum Incrementality {
    FULL_DATASET = 0;
    DIFFERENTIAL = 1;
  }
  optional Incrementality incrementality = 2 [default = FULL_DATASET];

  // This timestamp identifies the moment when the content of this feed has been
  // created (in server time). In POSIX time (i.e., number of seconds since
  // January 1st 1970 00:00:00 UTC).
  optional uint64 timestamp = 3;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// A definition (or update) of an entity in the transit feed.
message FeedEntity {
  // The ids are used only to provide incrementality support. The id should be
  // unique within a FeedMessage. Consequent FeedMessages may contain
  // FeedEntities with the same id. In case of a DIFFERENTIAL update the new
  // FeedEntity with some id will replace the old FeedEntity with the same id
  // (or delete it - see is_deleted below).
  // The actual GTFS entities (e.g. stations, routes, trips) referenced by the
  // feed must be specified by explicit selectors (see EntitySelector below for
  // more info).
  required string id = 1;

  // Whether this entity is to be deleted. Relevant only for incremental
  // fetches.
  optional bool is_deleted = 2 [default = false];

  // Data about the entity itself. Exactly one of the following fields must be
  // present (unless the entity is being deleted).
  optional TripUpdate trip_update = 3;
  optional VehiclePosition vehicle = 4;
  optional Alert alert = 5;

  // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
  optional Shape shape = 6;
  optional Stop stop = 7;
  optional TripModifications trip_modifications = 8;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

//
// Entities used in the feed.
//

// Realtime update of the progress of a vehicle along a trip.
// Depending on the value of ScheduleRelationship, a TripUpdate can specify:
// - A trip that proceeds along the schedule.
// - A trip that proceeds along a route but has no fixed schedule.
// - A trip that have been added or removed with regard to schedule.
//
// The updates can be for future, predicted arrival/departure events, or for
// past events that already occurred.
// Normally, updates should get more precise and more certain (see
// uncertainty below) as the events gets closer to current time.
// Even if that is not possible, the information for past events should be
// precise and certain. In particular, if an update points to time in the past
// but its update's uncertainty is not 0, the client should conclude that the
// update is a (wrong) prediction and that the trip has not completed yet.
//
// Note that the update can describe a trip that is already completed.
// To this end, it is enough to provide an update for the last stop of the trip.
// If the time of that is in the past, the client will conclude from that that
// the whole trip is in the past (it is possible, although inconsequential, to
// also provide updates for preceding stops).
// This option is most relevant for a trip that has completed ahead of schedule,
// but according to the schedule, the trip is still proceeding at the current
// time. Removing the updates for this trip could make the client assume
// that the trip is still proceeding.
// Note that the feed provider is allowed, but not required, to purge past
// updates - this is one case where this would be practically useful.
message TripUpdate {
  // The Trip that this message applies to. There can be at most one
  // TripUpdate entity for each actual trip instance.
  // If there is none, that means there is no prediction information available.
  // It does *not* mean that the trip is progressing according to schedule.
  required TripDescriptor trip = 1;

  // Additional information on the vehicle that is serving this trip.
  optional VehicleDescriptor vehicle = 3;

  // Timing information for a single predicted event (either arrival or
  // departure).
  // Timing consists of delay and/or estimated time, and uncertainty.
  // - delay should be used when the prediction is given relative to some
  //   existing schedule in GTFS.
  // - time should be given whether there is a predicted schedule or not. If
  //   both time and delay are specified, time will take precedence
  //   (although normally, time, if given for a scheduled trip, should be
  //   equal to scheduled time in GTFS + delay).
  //
  // Uncertainty applies equally to both time and delay.
  // The uncertainty roughly specifies the expected error in true delay (but
  // note, we don't yet define its precise statistical meaning). It's possible
  // for the uncertainty to be 0, for example for trains that are driven under
  // computer timing control.
  message StopTimeEvent {
    // Delay (in seconds) can be positive (meaning that the vehicle is late) or
    // negative (meaning that the vehicle is ahead of schedule). Delay of 0
    // means that the vehicle is exactly on time.
    optional int32 delay = 1;

    // Event as absolute time.
    // In Unix time (i.e., number of seconds since January 1st 1970 00:00:00
    // UTC).
    optional int64 time = 2;

    // If uncertainty is omitted, it is interpreted as unknown.
    // If the prediction is unknown or too uncertain, the delay (or time) field
    // should be empty. In such case, the uncertainty field is ignored.
    // To specify a completely certain prediction, set its uncertainty to 0.
    optional int32 uncertainty = 3;

    // The extensions namespace allows 3rd-party developers to extend the
    // GTFS Realtime Specification in order to add and evaluate new features
    // and modifications to the spec.
    extensions 1000 to 1999;

    // The following extension IDs are reserved for private use by any organization.
    extensions 9000 to 9999;
  }

  // Realtime update for arrival and/or departure events for a given stop on a
  // trip. Updates can be supplied for both past and future events.
  // The producer is allowed, although not required, to drop past events.
  message StopTimeUpdate {
    // The update is linked to a specific stop either through stop_sequence or
    // stop_id, so one of the fields below must necessarily be set.
    // See the documentation in TripDescriptor for more information.

    // Must be the same as in stop_times.txt in the corresponding GTFS feed.
    optional uint32 stop_sequence = 1;
    // Must be the same as in stops.txt in the corresponding GTFS feed.
    optional string stop_id = 4;

    optional StopTimeEvent arrival = 2;
    optional StopTimeEvent departure = 3;

    // Expected occupancy after departure from the given stop.
    // Should be provided only for future stops.
    // In order to provide departure_occupancy_status without either arrival or
    // departure StopTimeEvents, ScheduleRelationship should be set to NO_DATA. 
    optional VehiclePosition.OccupancyStatus departure_occupancy_status = 7;
    
    // The relation between the StopTimeEvents and the static schedule.
    enum ScheduleRelationship {
      // The vehicle is proceeding in accordance with its static schedule of
      // stops, although not necessarily according to the times of the schedule.
      // At least one of arrival and departure must be provided. If the schedule
      // for this stop contains both arrival and departure times then so must
      // this update. Frequency-based trips (GTFS frequencies.txt with exact_times = 0)
      // should not have a SCHEDULED value and should use UNSCHEDULED instead.
      SCHEDULED = 0;

      // The stop is skipped, i.e., the vehicle will not stop at this stop.
      // Arrival and departure are optional.
      SKIPPED = 1;

      // No StopTimeEvents are given for this stop.
      // The main intention for this value is to give time predictions only for
      // part of a trip, i.e., if the last update for a trip has a NO_DATA
      // specifier, then StopTimeEvents for the rest of the stops in the trip
      // are considered to be unspecified as well.
      // Neither arrival nor departure should be supplied.
      NO_DATA = 2;

      // The vehicle is operating a trip defined in GTFS frequencies.txt with exact_times = 0.
      // This value should not be used for trips that are not defined in GTFS frequencies.txt,
      // or trips in GTFS frequencies.txt with exact_times = 1. Trips containing StopTimeUpdates
      // with ScheduleRelationship=UNSCHEDULED must also set TripDescriptor.ScheduleRelationship=UNSCHEDULED.
      // NOTE: This field is still experimental, and subject to change. It may be
      // formally adopted in the future.
      UNSCHEDULED = 3;
    }
    optional ScheduleRelationship schedule_relationship = 5
        [default = SCHEDULED];

    // Provides the updated values for the stop time.
    // NOTE: This message is still experimental, and subject to change. It may be formally adopted in the future.
    message StopTimeProperties {
      // Supports real-time stop assignments. Refers to a stop_id defined in the GTFS stops.txt.
      // The new assigned_stop_id should not result in a significantly different trip experience for the end user than
      // the stop_id defined in GTFS stop_times.txt. In other words, the end user should not view this new stop_id as an
      // "unusual change" if the new stop was presented within an app without any additional context.
      // For example, this field is intended to be used for platform assignments by using a stop_id that belongs to the
      // same station as the stop originally defined in GTFS stop_times.txt.
      // To assign a stop without providing any real-time arrival or departure predictions, populate this field and set
      // StopTimeUpdate.schedule_relationship = NO_DATA.
      // If this field is populated, it is preferred to omit `StopTimeUpdate.stop_id` and use only `StopTimeUpdate.stop_sequence`. If
      // `StopTimeProperties.assigned_stop_id` and `StopTimeUpdate.stop_id` are populated, `StopTimeUpdate.stop_id` must match `assigned_stop_id`.
      // Platform assignments should be reflected in other GTFS-realtime fields as well
      // (e.g., `VehiclePosition.stop_id`).
      // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
      optional string assigned_stop_id = 1;

      // The extensions namespace allows 3rd-party developers to extend the
      // GTFS Realtime Specification in order to add and evaluate new features
      // and modifications to the spec.
      extensions 1000 to 1999;

      // The following extension IDs are reserved for private use by any organization.
      extensions 9000 to 9999;
    }

    // Realtime updates for certain properties defined within GTFS stop_times.txt
    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
    optional StopTimeProperties stop_time_properties = 6;

    // The extensions namespace allows 3rd-party developers to extend the
    // GTFS Realtime Specification in order to add and evaluate new features
    // and modifications to the spec.
    extensions 1000 to 1999;

    // The following extension IDs are reserved for private use by any organization.
    extensions 9000 to 9999;
  }

  // Updates to StopTimes for the trip (both future, i.e., predictions, and in
  // some cases, past ones, i.e., those that already happened).
  // The updates must be sorted by stop_sequence, and apply for all the
  // following stops of the trip up to the next specified one.
  //
  // Example 1:
  // For a trip with 20 stops, a StopTimeUpdate with arrival delay and departure
  // delay of 0 for stop_sequence of the current stop means that the trip is
  // exactly on time.
  //
  // Example 2:
  // For the same trip instance, 3 StopTimeUpdates are provided:
  // - delay of 5 min for stop_sequence 3
  // - delay of 1 min for stop_sequence 8
  // - delay of unspecified duration for stop_sequence 10
  // This will be interpreted as:
  // - stop_sequences 3,4,5,6,7 have delay of 5 min.
  // - stop_sequences 8,9 have delay of 1 min.
  // - stop_sequences 10,... have unknown delay.
  repeated StopTimeUpdate stop_time_update = 2;

  // The most recent moment at which the vehicle's real-time progress was measured
  // to estimate StopTimes in the future. When StopTimes in the past are provided,
  // arrival/departure times may be earlier than this value. In POSIX
  // time (i.e., the number of seconds since January 1st 1970 00:00:00 UTC).
  optional uint64 timestamp = 4;

  // The current schedule deviation for the trip.  Delay should only be
  // specified when the prediction is given relative to some existing schedule
  // in GTFS.
  //
  // Delay (in seconds) can be positive (meaning that the vehicle is late) or
  // negative (meaning that the vehicle is ahead of schedule). Delay of 0
  // means that the vehicle is exactly on time.
  //
  // Delay information in StopTimeUpdates take precedent of trip-level delay
  // information, such that trip-level delay is only propagated until the next
  // stop along the trip with a StopTimeUpdate delay value specified.
  //
  // Feed providers are strongly encouraged to provide a TripUpdate.timestamp
  // value indicating when the delay value was last updated, in order to
  // evaluate the freshness of the data.
  //
  // NOTE: This field is still experimental, and subject to change. It may be
  // formally adopted in the future.
  optional int32 delay = 5;

  // Defines updated properties of the trip, such as a new shape_id when there is a detour. Or defines the
  // trip_id, start_date, and start_time of a DUPLICATED trip. 
  // NOTE: This message is still experimental, and subject to change. It may be formally adopted in the future.
  message TripProperties {
    // Defines the identifier of a new trip that is a duplicate of an existing trip defined in (CSV) GTFS trips.txt
    // but will start at a different service date and/or time (defined using the TripProperties.start_date and
    // TripProperties.start_time fields). See definition of trips.trip_id in (CSV) GTFS. Its value must be different
    // than the ones used in the (CSV) GTFS. Required if schedule_relationship=DUPLICATED, otherwise this field must not
    // be populated and will be ignored by consumers.
    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
    optional string trip_id = 1;
    // Service date on which the DUPLICATED trip will be run, in YYYYMMDD format. Required if
    // schedule_relationship=DUPLICATED, otherwise this field must not be populated and will be ignored by consumers.
    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
    optional string start_date = 2;
    // Defines the departure start time of the trip when it’s duplicated. See definition of stop_times.departure_time
    // in (CSV) GTFS. Scheduled arrival and departure times for the duplicated trip are calculated based on the offset
    // between the original trip departure_time and this field. For example, if a GTFS trip has stop A with a
    // departure_time of 10:00:00 and stop B with departure_time of 10:01:00, and this field is populated with the value
    // of 10:30:00, stop B on the duplicated trip will have a scheduled departure_time of 10:31:00. Real-time prediction
    // delay values are applied to this calculated schedule time to determine the predicted time. For example, if a
    // departure delay of 30 is provided for stop B, then the predicted departure time is 10:31:30. Real-time
    // prediction time values do not have any offset applied to them and indicate the predicted time as provided.
    // For example, if a departure time representing 10:31:30 is provided for stop B, then the predicted departure time
    // is 10:31:30. This field is required if schedule_relationship is DUPLICATED, otherwise this field must not be
    // populated and will be ignored by consumers.
    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
    optional string start_time = 3;
    // Specifies the shape of the vehicle travel path when the trip shape differs from the shape specified in
    // (CSV) GTFS or to specify it in real-time when it's not provided by (CSV) GTFS, such as a vehicle that takes differing
    // paths based on rider demand. See definition of trips.shape_id in (CSV) GTFS. If a shape is neither defined in (CSV) GTFS
    // nor in real-time, the shape is considered unknown. This field can refer to a shape defined in the (CSV) GTFS in shapes.txt
    // or a Shape in the (protobuf) real-time feed. The order of stops (stop sequences) for this trip must remain the same as
    // (CSV) GTFS. Stops that are a part of the original trip but will no longer be made, such as when a detour occurs, should
    // be marked as schedule_relationship=SKIPPED.
    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future. 
    optional string shape_id = 4;

    // The extensions namespace allows 3rd-party developers to extend the
    // GTFS Realtime Specification in order to add and evaluate new features
    // and modifications to the spec.
    extensions 1000 to 1999;

    // The following extension IDs are reserved for private use by any organization.
    extensions 9000 to 9999;
  }
  optional TripProperties trip_properties = 6;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// Realtime positioning information for a given vehicle.
message VehiclePosition {
  // The Trip that this vehicle is serving.
  // Can be empty or partial if the vehicle can not be identified with a given
  // trip instance.
  optional TripDescriptor trip = 1;

  // Additional information on the vehicle that is serving this trip.
  optional VehicleDescriptor vehicle = 8;

  // Current position of this vehicle.
  optional Position position = 2;

  // The stop sequence index of the current stop. The meaning of
  // current_stop_sequence (i.e., the stop that it refers to) is determined by
  // current_status.
  // If current_status is missing IN_TRANSIT_TO is assumed.
  optional uint32 current_stop_sequence = 3;
  // Identifies the current stop. The value must be the same as in stops.txt in
  // the corresponding GTFS feed.
  optional string stop_id = 7;

  enum VehicleStopStatus {
    // The vehicle is just about to arrive at the stop (on a stop
    // display, the vehicle symbol typically flashes).
    INCOMING_AT = 0;

    // The vehicle is standing at the stop.
    STOPPED_AT = 1;

    // The vehicle has departed and is in transit to the next stop.
    IN_TRANSIT_TO = 2;
  }
  // The exact status of the vehicle with respect to the current stop.
  // Ignored if current_stop_sequence is missing.
  optional VehicleStopStatus current_status = 4 [default = IN_TRANSIT_TO];

  // Moment at which the vehicle's position was measured. In POSIX time
  // (i.e., number of seconds since January 1st 1970 00:00:00 UTC).
  optional uint64 timestamp = 5;

  // Congestion level that is affecting this vehicle.
  enum CongestionLevel {
    UNKNOWN_CONGESTION_LEVEL = 0;
    RUNNING_SMOOTHLY = 1;
    STOP_AND_GO = 2;
    CONGESTION = 3;
    SEVERE_CONGESTION = 4;  // People leaving their cars.
  }
  optional CongestionLevel congestion_level = 6;

  // The state of passenger occupancy for the vehicle or carriage.
  // Individual producers may not publish all OccupancyStatus values. Therefore, consumers
  // must not assume that the OccupancyStatus values follow a linear scale.
  // Consumers should represent OccupancyStatus values as the state indicated 
  // and intended by the producer. Likewise, producers must use OccupancyStatus values that
  // correspond to actual vehicle occupancy states.
  // For describing passenger occupancy levels on a linear scale, see `occupancy_percentage`.
  // This field is still experimental, and subject to change. It may be formally adopted in the future.
  enum OccupancyStatus {
    // The vehicle or carriage is considered empty by most measures, and has few or no
    // passengers onboard, but is still accepting passengers.
    EMPTY = 0;

    // The vehicle or carriage has a large number of seats available.
    // The amount of free seats out of the total seats available to be
    // considered large enough to fall into this category is determined at the
    // discretion of the producer.
    MANY_SEATS_AVAILABLE = 1;

    // The vehicle or carriage has a relatively small number of seats available.
    // The amount of free seats out of the total seats available to be
    // considered small enough to fall into this category is determined at the
    // discretion of the feed producer.
    FEW_SEATS_AVAILABLE = 2;

    // The vehicle or carriage can currently accommodate only standing passengers.
    STANDING_ROOM_ONLY = 3;

    // The vehicle or carriage can currently accommodate only standing passengers
    // and has limited space for them.
    CRUSHED_STANDING_ROOM_ONLY = 4;

    // The vehicle or carriage is considered full by most measures, but may still be
    // allowing passengers to board.
    FULL = 5;

    // The vehicle or carriage is not accepting passengers, but usually accepts passengers for boarding.
    NOT_ACCEPTING_PASSENGERS = 6;

    // The vehicle or carriage doesn't have any occupancy data available at that time.
    NO_DATA_AVAILABLE = 7;

    // The vehicle or carriage is not boardable and never accepts passengers.
    // Useful for special vehicles or carriages (engine, maintenance carriage, etc…).
    NOT_BOARDABLE = 8;

  }
  // If multi_carriage_status is populated with per-carriage OccupancyStatus,
  // then this field should describe the entire vehicle with all carriages accepting passengers considered.
  optional OccupancyStatus occupancy_status = 9;

  // A percentage value indicating the degree of passenger occupancy in the vehicle.
  // The values are represented as an integer without decimals. 0 means 0% and 100 means 100%.
  // The value 100 should represent the total maximum occupancy the vehicle was designed for,
  // including both seated and standing capacity, and current operating regulations allow.
  // The value may exceed 100 if there are more passengers than the maximum designed capacity.
  // The precision of occupancy_percentage should be low enough that individual passengers cannot be tracked boarding or alighting the vehicle.
  // If multi_carriage_status is populated with per-carriage occupancy_percentage, 
  // then this field should describe the entire vehicle with all carriages accepting passengers considered.
  // This field is still experimental, and subject to change. It may be formally adopted in the future.
  optional uint32 occupancy_percentage = 10;

  // Carriage specific details, used for vehicles composed of several carriages
  // This message/field is still experimental, and subject to change. It may be formally adopted in the future.
  message CarriageDetails {

    // Identification of the carriage. Should be unique per vehicle.
    optional string id = 1;

    // User visible label that may be shown to the passenger to help identify
    // the carriage. Example: "7712", "Car ABC-32", etc...
    // This message/field is still experimental, and subject to change. It may be formally adopted in the future.
    optional string label = 2;

    // Occupancy status for this given carriage, in this vehicle
    // This message/field is still experimental, and subject to change. It may be formally adopted in the future.
    optional OccupancyStatus occupancy_status = 3 [default = NO_DATA_AVAILABLE];

    // Occupancy percentage for this given carriage, in this vehicle.
    // Follows the same rules as "VehiclePosition.occupancy_percentage"
    // -1 in case data is not available for this given carriage (as protobuf defaults to 0 otherwise)
    // This message/field is still experimental, and subject to change. It may be formally adopted in the future.
    optional int32 occupancy_percentage = 4 [default = -1];

    // Identifies the order of this carriage with respect to the other
    // carriages in the vehicle's list of CarriageDetails.
    // The first carriage in the direction of travel must have a value of 1.
    // The second value corresponds to the second carriage in the direction
    // of travel and must have a value of 2, and so forth.
    // For example, the first carriage in the direction of travel has a value of 1.
    // If the second carriage in the direction of travel has a value of 3,
    // consumers will discard data for all carriages (i.e., the multi_carriage_details field).
    // Carriages without data must be represented with a valid carriage_sequence number and the fields 
    // without data should be omitted (alternately, those fields could also be included and set to the "no data" values).
    // This message/field is still experimental, and subject to change. It may be formally adopted in the future.
    optional uint32 carriage_sequence = 5;

    // The extensions namespace allows 3rd-party developers to extend the
    // GTFS Realtime Specification in order to add and evaluate new features and
    // modifications to the spec.
    extensions 1000 to 1999;

    // The following extension IDs are reserved for private use by any organization.
    extensions 9000 to 9999;
  }

  // Details of the multiple carriages of this given vehicle.
  // The first occurrence represents the first carriage of the vehicle, 
  // given the current direction of travel. 
  // The number of occurrences of the multi_carriage_details 
  // field represents the number of carriages of the vehicle.
  // It also includes non boardable carriages, 
  // like engines, maintenance carriages, etc… as they provide valuable 
  // information to passengers about where to stand on a platform.
  // This message/field is still experimental, and subject to change. It may be formally adopted in the future.
  repeated CarriageDetails multi_carriage_details = 11;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// An alert, indicating some sort of incident in the public transit network.
message Alert {
  // Time when the alert should be shown to the user. If missing, the
  // alert will be shown as long as it appears in the feed.
  // If multiple ranges are given, the alert will be shown during all of them.
  repeated TimeRange active_period = 1;

  // Entities whose users we should notify of this alert.
  repeated EntitySelector informed_entity = 5;

  // Cause of this alert. If cause_detail is included, then Cause must also be included.
  enum Cause {
    UNKNOWN_CAUSE = 1;
    OTHER_CAUSE = 2;        // Not machine-representable.
    TECHNICAL_PROBLEM = 3;
    STRIKE = 4;             // Public transit agency employees stopped working.
    DEMONSTRATION = 5;      // People are blocking the streets.
    ACCIDENT = 6;
    HOLIDAY = 7;
    WEATHER = 8;
    MAINTENANCE = 9;
    CONSTRUCTION = 10;
    POLICE_ACTIVITY = 11;
    MEDICAL_EMERGENCY = 12;
  }
  optional Cause cause = 6 [default = UNKNOWN_CAUSE];

  // What is the effect of this problem on the affected entity. If effect_detail is included, then Effect must also be included.
  enum Effect {
    NO_SERVICE = 1;
    REDUCED_SERVICE = 2;

    // We don't care about INsignificant delays: they are hard to detect, have
    // little impact on the user, and would clutter the results as they are too
    // frequent.
    SIGNIFICANT_DELAYS = 3;

    DETOUR = 4;
    ADDITIONAL_SERVICE = 5;
    MODIFIED_SERVICE = 6;
    OTHER_EFFECT = 7;
    UNKNOWN_EFFECT = 8;
    STOP_MOVED = 9;
    NO_EFFECT = 10;
    ACCESSIBILITY_ISSUE = 11;
  }
  optional Effect effect = 7 [default = UNKNOWN_EFFECT];

  // The URL which provides additional information about the alert.
  optional TranslatedString url = 8;

  // Alert header. Contains a short summary of the alert text as plain-text.
  optional TranslatedString header_text = 10;

  // Full description for the alert as plain-text. The information in the
  // description should add to the information of the header.
  optional TranslatedString description_text = 11;

  // Text for alert header to be used in text-to-speech implementations. This field is the text-to-speech version of header_text.
  optional TranslatedString tts_header_text = 12;

  // Text for full description for the alert to be used in text-to-speech implementations. This field is the text-to-speech version of description_text.
  optional TranslatedString tts_description_text = 13;

  // Severity of this alert.
  enum SeverityLevel {
	UNKNOWN_SEVERITY = 1;
	INFO = 2;
	WARNING = 3;
	SEVERE = 4;
  }

  optional SeverityLevel severity_level = 14 [default = UNKNOWN_SEVERITY];

  // TranslatedImage to be displayed along the alert text. Used to explain visually the alert effect of a detour, station closure, etc. The image must enhance the understanding of the alert. Any essential information communicated within the image must also be contained in the alert text.
  // The following types of images are discouraged : image containing mainly text, marketing or branded images that add no additional information. 
  // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
  optional TranslatedImage image = 15; 

  // Text describing the appearance of the linked image in the `image` field (e.g., in case the image can't be displayed
  // or the user can't see the image for accessibility reasons). See the HTML spec for alt image text - https://html.spec.whatwg.org/#alt.
  // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
  optional TranslatedString image_alternative_text = 16;
  
  
  // Description of the cause of the alert that allows for agency-specific language; more specific than the Cause. If cause_detail is included, then Cause must also be included.
  // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
  optional TranslatedString cause_detail = 17;
  
  // Description of the effect of the alert that allows for agency-specific language; more specific than the Effect. If effect_detail is included, then Effect must also be included.
  // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
  optional TranslatedString effect_detail = 18;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features
  // and modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

//
// Low level data structures used above.
//

// A time interval. The interval is considered active at time 't' if 't' is
// greater than or equal to the start time and less than the end time.
message TimeRange {
  // Start time, in POSIX time (i.e., number of seconds since January 1st 1970
  // 00:00:00 UTC).
  // If missing, the interval starts at minus infinity.
  optional uint64 start = 1;

  // End time, in POSIX time (i.e., number of seconds since January 1st 1970
  // 00:00:00 UTC).
  // If missing, the interval ends at plus infinity.
  optional uint64 end = 2;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// A position.
message Position {
  // Degrees North, in the WGS-84 coordinate system.
  required float latitude = 1;

  // Degrees East, in the WGS-84 coordinate system.
  required float longitude = 2;

  // Bearing, in degrees, clockwise from North, i.e., 0 is North and 90 is East.
  // This can be the compass bearing, or the direction towards the next stop
  // or intermediate location.
  // This should not be direction deduced from the sequence of previous
  // positions, which can be computed from previous data.
  optional float bearing = 3;

  // Odometer value, in meters.
  optional double odometer = 4;
  // Momentary speed measured by the vehicle, in meters per second.
  optional float speed = 5;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// A descriptor that identifies an instance of a GTFS trip, or all instances of
// a trip along a route.
// - To specify a single trip instance, the trip_id (and if necessary,
//   start_time) is set. If route_id is also set, then it should be same as one
//   that the given trip corresponds to.
// - To specify all the trips along a given route, only the route_id should be
//   set. Note that if the trip_id is not known, then stop sequence ids in
//   TripUpdate are not sufficient, and stop_ids must be provided as well. In
//   addition, absolute arrival/departure times must be provided.
message TripDescriptor {
  // The trip_id from the GTFS feed that this selector refers to.
  // For non frequency-based trips, this field is enough to uniquely identify
  // the trip. For frequency-based trip, start_time and start_date might also be
  // necessary. When schedule_relationship is DUPLICATED within a TripUpdate, the trip_id identifies the trip from
  // static GTFS to be duplicated. When schedule_relationship is DUPLICATED within a VehiclePosition, the trip_id
  // identifies the new duplicate trip and must contain the value for the corresponding TripUpdate.TripProperties.trip_id.
  optional string trip_id = 1;

  // The route_id from the GTFS that this selector refers to.
  optional string route_id = 5;

  // The direction_id from the GTFS feed trips.txt file, indicating the
  // direction of travel for trips this selector refers to.
  optional uint32 direction_id = 6;

  // The initially scheduled start time of this trip instance.
  // When the trip_id corresponds to a non-frequency-based trip, this field
  // should either be omitted or be equal to the value in the GTFS feed. When
  // the trip_id correponds to a frequency-based trip, the start_time must be
  // specified for trip updates and vehicle positions. If the trip corresponds
  // to exact_times=1 GTFS record, then start_time must be some multiple
  // (including zero) of headway_secs later than frequencies.txt start_time for
  // the corresponding time period. If the trip corresponds to exact_times=0,
  // then its start_time may be arbitrary, and is initially expected to be the
  // first departure of the trip. Once established, the start_time of this
  // frequency-based trip should be considered immutable, even if the first
  // departure time changes -- that time change may instead be reflected in a
  // StopTimeUpdate.
  // Format and semantics of the field is same as that of
  // GTFS/frequencies.txt/start_time, e.g., 11:15:35 or 25:15:35.
  optional string start_time = 2;
  // The scheduled start date of this trip instance.
  // Must be provided to disambiguate trips that are so late as to collide with
  // a scheduled trip on a next day. For example, for a train that departs 8:00
  // and 20:00 every day, and is 12 hours late, there would be two distinct
  // trips on the same time.
  // This field can be provided but is not mandatory for schedules in which such
  // collisions are impossible - for example, a service running on hourly
  // schedule where a vehicle that is one hour late is not considered to be
  // related to schedule anymore.
  // In YYYYMMDD format.
  optional string start_date = 3;

  // The relation between this trip and the static schedule. If a trip is done
  // in accordance with temporary schedule, not reflected in GTFS, then it
  // shouldn't be marked as SCHEDULED, but likely as ADDED.
  enum ScheduleRelationship {
    // Trip that is running in accordance with its GTFS schedule, or is close
    // enough to the scheduled trip to be associated with it.
    SCHEDULED = 0;

    // An extra trip that was added in addition to a running schedule, for
    // example, to replace a broken vehicle or to respond to sudden passenger
    // load.
    // NOTE: Currently, behavior is unspecified for feeds that use this mode. There are discussions on the GTFS GitHub
    // [(1)](https://github.com/google/transit/issues/106) [(2)](https://github.com/google/transit/pull/221)
    // [(3)](https://github.com/google/transit/pull/219) around fully specifying or deprecating ADDED trips and the
    // documentation will be updated when those discussions are finalized.
    ADDED = 1;

    // A trip that is running with no schedule associated to it (GTFS frequencies.txt exact_times=0).
    // Trips with ScheduleRelationship=UNSCHEDULED must also set all StopTimeUpdates.ScheduleRelationship=UNSCHEDULED.
    UNSCHEDULED = 2;

    // A trip that existed in the schedule but was removed.
    CANCELED = 3;

    // Should not be used - for backwards-compatibility only.
    REPLACEMENT = 5 [deprecated=true];

    // An extra trip that was added in addition to a running schedule, for example, to replace a broken vehicle or to
    // respond to sudden passenger load. Used with TripUpdate.TripProperties.trip_id, TripUpdate.TripProperties.start_date,
    // and TripUpdate.TripProperties.start_time to copy an existing trip from static GTFS but start at a different service
    // date and/or time. Duplicating a trip is allowed if the service related to the original trip in (CSV) GTFS
    // (in calendar.txt or calendar_dates.txt) is operating within the next 30 days. The trip to be duplicated is
    // identified via TripUpdate.TripDescriptor.trip_id. This enumeration does not modify the existing trip referenced by
    // TripUpdate.TripDescriptor.trip_id - if a producer wants to cancel the original trip, it must publish a separate
    // TripUpdate with the value of CANCELED or DELETED. Trips defined in GTFS frequencies.txt with exact_times that is
    // empty or equal to 0 cannot be duplicated. The VehiclePosition.TripDescriptor.trip_id for the new trip must contain
    // the matching value from TripUpdate.TripProperties.trip_id and VehiclePosition.TripDescriptor.ScheduleRelationship
    // must also be set to DUPLICATED.
    // Existing producers and consumers that were using the ADDED enumeration to represent duplicated trips must follow
    // the migration guide (https://github.com/google/transit/tree/master/gtfs-realtime/spec/en/examples/migration-duplicated.md)
    // to transition to the DUPLICATED enumeration.
    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
    DUPLICATED = 6;


    // A trip that existed in the schedule but was removed and must not be shown to users.
    // DELETED should be used instead of CANCELED to indicate that a transit provider would like to entirely remove
    // information about the corresponding trip from consuming applications, so the trip is not shown as cancelled to
    // riders, e.g. a trip that is entirely being replaced by another trip.
    // This designation becomes particularly important if several trips are cancelled and replaced with substitute service.
    // If consumers were to show explicit information about the cancellations it would distract from the more important
    // real-time predictions.
    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
    DELETED = 7;
  }
  optional ScheduleRelationship schedule_relationship = 4;

  message ModifiedTripSelector {
    // The 'id' from the FeedEntity in which the contained TripModifications object affects this trip.
    optional string modifications_id = 1;

    // The trip_id from the GTFS feed that is modified by the modifications_id
    optional string affected_trip_id = 2;
  }
  optional ModifiedTripSelector modified_trip = 7;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// Identification information for the vehicle performing the trip.
message VehicleDescriptor {
  // Internal system identification of the vehicle. Should be unique per
  // vehicle, and can be used for tracking the vehicle as it proceeds through
  // the system.
  optional string id = 1;

  // User visible label, i.e., something that must be shown to the passenger to
  // help identify the correct vehicle.
  optional string label = 2;

  // The license plate of the vehicle.
  optional string license_plate = 3;

  enum WheelchairAccessible {
    // The trip doesn't have information about wheelchair accessibility.
    // This is the **default** behavior. If the static GTFS contains a
    // _wheelchair_accessible_ value, it won't be overwritten.
    NO_VALUE = 0;

    // The trip has no accessibility value present.
    // This value will overwrite the value from the GTFS.
    UNKNOWN = 1;

    // The trip is wheelchair accessible.
    // This value will overwrite the value from the GTFS.
    WHEELCHAIR_ACCESSIBLE = 2;

    // The trip is **not** wheelchair accessible.
    // This value will overwrite the value from the GTFS.
    WHEELCHAIR_INACCESSIBLE = 3;
  }
  optional WheelchairAccessible wheelchair_accessible = 4 [default = NO_VALUE];

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// A selector for an entity in a GTFS feed.
message EntitySelector {
  // The values of the fields should correspond to the appropriate fields in the
  // GTFS feed.
  // At least one specifier must be given. If several are given, then the
  // matching has to apply to all the given specifiers.
  optional string agency_id = 1;
  optional string route_id = 2;
  // corresponds to route_type in GTFS.
  optional int32 route_type = 3;
  optional TripDescriptor trip = 4;
  optional string stop_id = 5;
  // Corresponds to trip direction_id in GTFS trips.txt. If provided the
  // route_id must also be provided.
  optional uint32 direction_id = 6;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// An internationalized message containing per-language versions of a snippet of
// text or a URL.
// One of the strings from a message will be picked up. The resolution proceeds
// as follows:
// 1. If the UI language matches the language code of a translation,
//    the first matching translation is picked.
// 2. If a default UI language (e.g., English) matches the language code of a
//    translation, the first matching translation is picked.
// 3. If some translation has an unspecified language code, that translation is
//    picked.
message TranslatedString {
  message Translation {
    // A UTF-8 string containing the message.
    required string text = 1;
    // BCP-47 language code. Can be omitted if the language is unknown or if
    // no i18n is done at all for the feed. At most one translation is
    // allowed to have an unspecified language tag.
    optional string language = 2;

    // The extensions namespace allows 3rd-party developers to extend the
    // GTFS Realtime Specification in order to add and evaluate new features and
    // modifications to the spec.
    extensions 1000 to 1999;

    // The following extension IDs are reserved for private use by any organization.
    extensions 9000 to 9999;
  }
  // At least one translation must be provided.
  repeated Translation translation = 1;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// An internationalized image containing per-language versions of a URL linking to an image
// along with meta information
// Only one of the images from a message will be retained by consumers. The resolution proceeds
// as follows:
// 1. If the UI language matches the language code of a translation,
//    the first matching translation is picked.
// 2. If a default UI language (e.g., English) matches the language code of a
//    translation, the first matching translation is picked.
// 3. If some translation has an unspecified language code, that translation is
//    picked.
// NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
message TranslatedImage {
  message LocalizedImage {
    // String containing an URL linking to an image
    // The image linked must be less than 2MB. 
    // If an image changes in a significant enough way that an update is required on the consumer side, the producer must update the URL to a new one.
    // The URL should be a fully qualified URL that includes http:// or https://, and any special characters in the URL must be correctly escaped. See the following http://www.w3.org/Addressing/URL/4_URI_Recommentations.html for a description of how to create fully qualified URL values.
    required string url = 1;

    // IANA media type as to specify the type of image to be displayed. 
    // The type must start with "image/"
    required string media_type = 2;

    // BCP-47 language code. Can be omitted if the language is unknown or if
    // no i18n is done at all for the feed. At most one translation is
    // allowed to have an unspecified language tag.
    optional string language = 3;


    // The extensions namespace allows 3rd-party developers to extend the
    // GTFS Realtime Specification in order to add and evaluate new features and
    // modifications to the spec.
    extensions 1000 to 1999;

    // The following extension IDs are reserved for private use by any organization.
    extensions 9000 to 9999;
  }
  // At least one localized image must be provided.
  repeated LocalizedImage localized_image = 1;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// Describes the physical path that a vehicle takes when it's not part of the (CSV) GTFS,
// such as for a detour. Shapes belong to Trips, and consist of a sequence of shape points.
// Tracing the points in order provides the path of the vehicle.  Shapes do not need to intercept
// the location of Stops exactly, but all Stops on a trip should lie within a small distance of
// the shape for that trip, i.e. close to straight line segments connecting the shape points
// NOTE: This message is still experimental, and subject to change. It may be formally adopted in the future.
message Shape {
  // Identifier of the shape. Must be different than any shape_id defined in the (CSV) GTFS.
  // This field is required as per reference.md, but needs to be specified here optional because "Required is Forever"
  // See https://developers.google.com/protocol-buffers/docs/proto#specifying_field_rules
  // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
  optional string shape_id = 1;
  
  // Encoded polyline representation of the shape. This polyline must contain at least two points.
  // For more information about encoded polylines, see https://developers.google.com/maps/documentation/utilities/polylinealgorithm
  // This field is required as per reference.md, but needs to be specified here optional because "Required is Forever"
  // See https://developers.google.com/protocol-buffers/docs/proto#specifying_field_rules
  // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
  optional string encoded_polyline = 2;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// Describes a stop which is served by trips. All fields are as described in the GTFS-Static specification.
// NOTE: This message is still experimental, and subject to change. It may be formally adopted in the future.
message Stop {
  enum WheelchairBoarding {
    UNKNOWN = 0;
    AVAILABLE = 1;
    NOT_AVAILABLE = 2;
   }

  optional string stop_id = 1;
  optional TranslatedString stop_code = 2;
  optional TranslatedString stop_name = 3;
  optional TranslatedString tts_stop_name = 4;
  optional TranslatedString stop_desc = 5;
  optional float stop_lat = 6;
  optional float stop_lon = 7;
  optional string zone_id = 8;
  optional TranslatedString stop_url = 9;
  optional string parent_station = 11;
  optional string stop_timezone = 12;
  optional WheelchairBoarding wheelchair_boarding = 13 [default = UNKNOWN];
  optional string level_id = 14;
  optional TranslatedString platform_code = 15;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
message TripModifications {
  // A `Modification` message replaces a span of n stop times from each affected trip starting at `start_stop_selector`.
  message Modification {
    // The stop selector of the first stop_time of the original trip that is to be affected by this modification.
    // Used in conjuction with `end_stop_selector`. 
    // `start_stop_selector` is required and is used to define the reference stop used with `travel_time_to_stop`.
    optional StopSelector start_stop_selector = 1;

    // The stop selector of the last stop of the original trip that is to be affected by this modification. 
    // The selection is inclusive, so if only one stop_time is replaced by that modification, `start_stop_selector` and `end_stop_selector` must be equivalent.
    // If no stop_time is replaced, `end_stop_selector` must not be provided. It's otherwise required.
    optional StopSelector end_stop_selector = 2;

    // The number of seconds of delay to add to all departure and arrival times following the end of this modification. 
    // If multiple modifications apply to the same trip, the delays accumulate as the trip advances. 
    optional int32 propagated_modification_delay = 3 [default = 0];

    // A list of replacement stops, replacing those of the original trip. 
    // The length of the new stop times may be less, the same, or greater than the number of replaced stop times. 
    repeated ReplacementStop replacement_stops = 4;

    // An `id` value from the `FeedEntity` message that contains the `Alert` describing this Modification for user-facing communication.
    optional string service_alert_id = 5;

    // This timestamp identifies the moment when the modification has last been changed.
    // In POSIX time (i.e., number of seconds since January 1st 1970 00:00:00 UTC).
    optional uint64 last_modified_time = 6;

    // The extensions namespace allows 3rd-party developers to extend the
    // GTFS Realtime Specification in order to add and evaluate new features and
    // modifications to the spec.
    extensions 1000 to 1999;

    // The following extension IDs are reserved for private use by any organization.
    extensions 9000 to 9999;
  }

  message SelectedTrips {
    // A list of trips affected with this replacement that all have the same new `shape_id`.
    repeated string trip_ids = 1;
    // The ID of the new shape for the modified trips in this SelectedTrips.
    // May refer to a new shape added using a GTFS-RT Shape message, or to an existing shape defined in the GTFS-Static feed’s shapes.txt.
    optional string shape_id = 2;

    // The extensions namespace allows 3rd-party developers to extend the
    // GTFS Realtime Specification in order to add and evaluate new features and
    // modifications to the spec.
    extensions 1000 to 1999;

    // The following extension IDs are reserved for private use by any organization.
    extensions 9000 to 9999;
}

   // A list of selected trips affected by this TripModifications.
  repeated SelectedTrips selected_trips = 1;

  // A list of start times in the real-time trip descriptor for the trip_id defined in trip_ids. 
  // Useful to target multiple departures of a trip_id in a frequency-based trip.
  repeated string start_times = 2;

  // Dates on which the modifications occurs, in the YYYYMMDD format. Producers SHOULD only transmit detours occurring within the next week.
  // The dates provided should not be used as user-facing information, if a user-facing start and end date needs to be provided, they can be provided in the linked service alert with `service_alert_id`
  repeated string service_dates = 3;

  // A list of modifications to apply to the affected trips. 
  repeated Modification modifications = 4; 

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
// Select a stop by stop sequence or by stop_id. At least one of the two values must be provided.
message StopSelector { 
  // Must be the same as in stop_times.txt in the corresponding GTFS feed.
  optional uint32 stop_sequence = 1;
  // Must be the same as in stops.txt in the corresponding GTFS feed.
  optional string stop_id = 2;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}

// NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
message ReplacementStop {
  // The difference in seconds between the arrival time at this stop and the arrival time at the reference stop. The reference stop is the stop prior to start_stop_selector. If the modification begins at the first stop of the trip, then the first stop of the trip is the reference stop.
  // This value MUST be monotonically increasing and may only be a negative number if the first stop of the original trip is the reference stop.
  optional int32 travel_time_to_stop = 1;

  // The replacement stop ID which will now be visited by the trip. May refer to a new stop added using a GTFS-RT Stop message, or to an existing stop defined in the GTFS-Static feed’s stops.txt. The stop MUST have location_type=0 (routable stops).
  optional string stop_id = 2;

  // The extensions namespace allows 3rd-party developers to extend the
  // GTFS Realtime Specification in order to add and evaluate new features and
  // modifications to the spec.
  extensions 1000 to 1999;

  // The following extension IDs are reserved for private use by any organization.
  extensions 9000 to 9999;
}