diff --git a/Sources/ContentView.swift b/Sources/ContentView.swift
index 698df576a5fc..9bf4d595f8f1 100644
--- a/Sources/ContentView.swift
+++ b/Sources/ContentView.swift
@@ -203,6 +203,22 @@ struct SidebarWorkspaceRowBackgroundStyle {
     static let clear = Self(color: nil, opacity: 0)
 }
 
+func sidebarWorkspaceRowExplicitRailNSColor(
+    activeTabIndicatorStyle: SidebarActiveTabIndicatorStyle,
+    customColorHex: String?,
+    colorScheme: ColorScheme
+) -> NSColor? {
+    guard activeTabIndicatorStyle == .leftRail,
+          let customColorHex else {
+        return nil
+    }
+    return WorkspaceTabColorSettings.displayNSColor(
+        hex: customColorHex,
+        colorScheme: colorScheme,
+        forceBright: true
+    )
+}
+
 func sidebarWorkspaceRowBackgroundStyle(
     activeTabIndicatorStyle: SidebarActiveTabIndicatorStyle,
     isActive: Bool,
@@ -223,24 +239,15 @@ func sidebarWorkspaceRowBackgroundStyle(
             forceBright: activeTabIndicatorStyle == .leftRail
         )
     }
-    let selectedCustomBackground = customColorHex.flatMap {
-        sidebarSelectedWorkspaceCustomBackgroundNSColor(hex: $0, colorScheme: colorScheme)
-    }
 
     switch activeTabIndicatorStyle {
     case .leftRail:
         if isActive {
             return SidebarWorkspaceRowBackgroundStyle(
-                color: selectedCustomBackground ?? selectedBackground,
+                color: selectedBackground,
                 opacity: 1
             )
         }
-        if let customBackground {
-            return SidebarWorkspaceRowBackgroundStyle(
-                color: customBackground,
-                opacity: isMultiSelected ? 0.35 : 0.7
-            )
-        }
         if isMultiSelected {
             return SidebarWorkspaceRowBackgroundStyle(color: accentBackground, opacity: 0.25)
         }
@@ -249,7 +256,7 @@ func sidebarWorkspaceRowBackgroundStyle(
     case .solidFill:
         if isActive {
             return SidebarWorkspaceRowBackgroundStyle(
-                color: selectedCustomBackground ?? selectedBackground,
+                color: selectedBackground,
                 opacity: 1
             )
         }
@@ -13073,6 +13080,10 @@ private struct TabItemView: View, Equatable {
         .semibold
     }
 
+    private var showsLeadingRail: Bool {
+        explicitRailColor != nil
+    }
+
     private var activeBorderLineWidth: CGFloat {
         switch activeTabIndicatorStyle {
         case .leftRail:
@@ -13501,6 +13512,16 @@ private struct TabItemView: View, Equatable {
                     RoundedRectangle(cornerRadius: 6)
                         .strokeBorder(activeBorderColor, lineWidth: activeBorderLineWidth)
                 }
+                .overlay(alignment: .leading) {
+                    if showsLeadingRail {
+                        Capsule(style: .continuous)
+                            .fill(railColor)
+                            .frame(width: 3)
+                            .padding(.leading, 4)
+                            .padding(.vertical, 5)
+                            .offset(x: -1)
+                    }
+                }
         )
         .padding(.horizontal, 6)
         .background {
@@ -13943,6 +13964,21 @@ private struct TabItemView: View, Equatable {
         return Color(nsColor: color).opacity(style.opacity)
     }
 
+    private var railColor: Color {
+        explicitRailColor ?? .clear
+    }
+
+    private var explicitRailColor: Color? {
+        guard let railColor = sidebarWorkspaceRowExplicitRailNSColor(
+            activeTabIndicatorStyle: activeTabIndicatorStyle,
+            customColorHex: workspaceSnapshot.customColorHex,
+            colorScheme: colorScheme
+        ) else {
+            return nil
+        }
+        return Color(nsColor: railColor).opacity(0.95)
+    }
+
     private func tabColorSwatchColor(for hex: String) -> NSColor {
         WorkspaceTabColorSettings.displayNSColor(
             hex: hex,
diff --git a/cmuxTests/WorkspaceUnitTests.swift b/cmuxTests/WorkspaceUnitTests.swift
index a4e36a7613a6..f6c3708f4f0c 100644
--- a/cmuxTests/WorkspaceUnitTests.swift
+++ b/cmuxTests/WorkspaceUnitTests.swift
@@ -61,7 +61,7 @@ final class SidebarSelectedWorkspaceColorTests: XCTestCase {
     }
 
     @MainActor
-    func testSetTabColorFeedsSelectedSolidFillSidebarBackground() {
+    func testSolidFillKeepsSelectedBackgroundForActiveCustomColoredWorkspaceRow() {
         let manager = TabManager()
         guard let workspace = manager.tabs.first else {
             XCTFail("Expected TabManager to initialise with a workspace")
@@ -87,13 +87,16 @@ final class SidebarSelectedWorkspaceColorTests: XCTestCase {
             sidebarSelectionColorHex: nil
         )
 
-        XCTAssertEqual(background.color?.hexString(), "#C0392B")
+        XCTAssertEqual(
+            background.color?.hexString(),
+            sidebarSelectedWorkspaceBackgroundNSColor(for: .light).hexString()
+        )
         XCTAssertEqual(background.opacity, 1.0, accuracy: 0.001)
         withExtendedLifetime(cancellable) {}
     }
 
     @MainActor
-    func testSetTabColorFeedsSelectedDefaultSidebarBackground() {
+    func testLeftRailKeepsSelectedBackgroundForActiveCustomColoredWorkspaceRow() {
         let manager = TabManager()
         guard let workspace = manager.tabs.first else {
             XCTFail("Expected TabManager to initialise with a workspace")
@@ -119,10 +122,79 @@ final class SidebarSelectedWorkspaceColorTests: XCTestCase {
             sidebarSelectionColorHex: nil
         )
 
-        XCTAssertEqual(background.color?.hexString(), "#C0392B")
+        XCTAssertEqual(
+            background.color?.hexString(),
+            sidebarSelectedWorkspaceBackgroundNSColor(for: .light).hexString()
+        )
         XCTAssertEqual(background.opacity, 1.0, accuracy: 0.001)
         withExtendedLifetime(cancellable) {}
     }
+
+    @MainActor
+    func testLeftRailLeavesInactiveCustomColoredWorkspaceRowTransparent() {
+        let manager = TabManager()
+        guard let workspace = manager.tabs.first else {
+            XCTFail("Expected TabManager to initialise with a workspace")
+            return
+        }
+
+        manager.setTabColor(tabId: workspace.id, color: "#C0392B")
+
+        let background = sidebarWorkspaceRowBackgroundStyle(
+            activeTabIndicatorStyle: .leftRail,
+            isActive: false,
+            isMultiSelected: false,
+            customColorHex: workspace.customColor,
+            colorScheme: .light,
+            sidebarSelectionColorHex: nil
+        )
+
+        XCTAssertNil(background.color)
+        XCTAssertEqual(background.opacity, 0, accuracy: 0.001)
+    }
+
+    @MainActor
+    func testLeftRailResolvesExplicitRailColorForCustomColoredWorkspaceRow() {
+        let manager = TabManager()
+        guard let workspace = manager.tabs.first else {
+            XCTFail("Expected TabManager to initialise with a workspace")
+            return
+        }
+
+        manager.setTabColor(tabId: workspace.id, color: "#C0392B")
+
+        let railColor = sidebarWorkspaceRowExplicitRailNSColor(
+            activeTabIndicatorStyle: .leftRail,
+            customColorHex: workspace.customColor,
+            colorScheme: .light
+        )
+
+        XCTAssertNotNil(railColor)
+        XCTAssertEqual(railColor?.hexString(), "#C0392B")
+    }
+
+    @MainActor
+    func testSolidFillUsesInactiveCustomWorkspaceColorAsBackground() {
+        let manager = TabManager()
+        guard let workspace = manager.tabs.first else {
+            XCTFail("Expected TabManager to initialise with a workspace")
+            return
+        }
+
+        manager.setTabColor(tabId: workspace.id, color: "#C0392B")
+
+        let background = sidebarWorkspaceRowBackgroundStyle(
+            activeTabIndicatorStyle: .solidFill,
+            isActive: false,
+            isMultiSelected: false,
+            customColorHex: workspace.customColor,
+            colorScheme: .light,
+            sidebarSelectionColorHex: nil
+        )
+
+        XCTAssertEqual(background.color?.hexString(), "#C0392B")
+        XCTAssertEqual(background.opacity, 0.7, accuracy: 0.001)
+    }
 }