diff --git a/gtfs-realtime/proto/gtfs-realtime.proto b/gtfs-realtime/proto/gtfs-realtime.proto
index 88581ab05..095436722 100644
--- a/gtfs-realtime/proto/gtfs-realtime.proto
+++ b/gtfs-realtime/proto/gtfs-realtime.proto
@@ -242,6 +242,37 @@ message TripUpdate {
     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.
diff --git a/gtfs-realtime/spec/en/reference.md b/gtfs-realtime/spec/en/reference.md
index 428442889..f1ed3d2f9 100644
--- a/gtfs-realtime/spec/en/reference.md
+++ b/gtfs-realtime/spec/en/reference.md
@@ -49,7 +49,8 @@ Fields labeled as **experimental** are subject to change and not yet formally ad
             *   [StopTimeUpdate](#message-stoptimeupdate)
                 *   [StopTimeEvent](#message-stoptimeevent)
                 *   [ScheduleRelationship](#enum-schedulerelationship)
-            *   [TripProperties](#message-tripproperties) 
+                *   [StopTimeProperties](#message-stoptimeproperties)
+            *   [TripProperties](#message-tripproperties)
         *   [VehiclePosition](#message-vehicleposition)
             *   [TripDescriptor](#message-tripdescriptor)
                 *   [ScheduleRelationship](#enum-schedulerelationship-1)
@@ -180,11 +181,12 @@ The update is linked to a specific stop either through stop_sequence or stop_id,
 
 | _**Field Name**_ | _**Type**_ | _**Required**_ | _**Cardinality**_ | _**Description**_ |
 |------------------|------------|----------------|-------------------|-------------------|
-| **stop_sequence** | [uint32](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Conditionally required | One | Must be the same as in stop_times.txt in the corresponding GTFS feed.  Either stop_sequence or stop_id must be provided within a StopTimeUpdate - both fields cannot be empty.  stop_sequence is required for trips that visit the same stop_id more than once (e.g., a loop) to disambiguate which stop the prediction is for. |
-| **stop_id** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Conditionally required | One | Must be the same as in stops.txt in the corresponding GTFS feed. Either stop_sequence or stop_id must be provided within a StopTimeUpdate - both fields cannot be empty. |
+| **stop_sequence** | [uint32](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Conditionally required | One | Must be the same as in stop_times.txt in the corresponding GTFS feed.  Either stop_sequence or stop_id must be provided within a StopTimeUpdate - both fields cannot be empty.  stop_sequence is required for trips that visit the same stop_id more than once (e.g., a loop) to disambiguate which stop the prediction is for. If `StopTimeProperties.assigned_stop_id` is populated, then `stop_sequence` must be populated. |
+| **stop_id** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Conditionally required | One | Must be the same as in stops.txt in the corresponding GTFS feed. Either stop_sequence or stop_id must be provided within a StopTimeUpdate - both fields cannot be empty. If `StopTimeProperties.assigned_stop_id` is populated, it is preferred to omit `stop_id` and use only `stop_sequence`. If `StopTimeProperties.assigned_stop_id` and `stop_id` are populated, `stop_id` must match `assigned_stop_id`. |
 | **arrival** | [StopTimeEvent](#message-stoptimeevent) | Conditionally required | One | If schedule_relationship is empty or SCHEDULED, either arrival or departure must be provided within a StopTimeUpdate - both fields cannot be empty. arrival and departure may both be empty when schedule_relationship is SKIPPED.  If schedule_relationship is NO_DATA, arrival and departure must be empty. |
 | **departure** | [StopTimeEvent](#message-stoptimeevent) | Conditionally required | One | If schedule_relationship is empty or SCHEDULED, either arrival or departure must be provided within a StopTimeUpdate - both fields cannot be empty. arrival and departure may both be empty when schedule_relationship is SKIPPED.  If schedule_relationship is NO_DATA, arrival and departure must be empty. |
 | **schedule_relationship** | [ScheduleRelationship](#enum-schedulerelationship) | Optional | One | The default relationship is SCHEDULED. |
+| **stop_time_properties** | [StopTimeProperties](#message-stoptimeproperties) | Optional | One | Realtime updates for certain properties defined within GTFS stop_times.txt <br>**Caution:** this field is still **experimental**, and subject to change. It may be formally adopted in the future.<br>. |
 
 ## _enum_ ScheduleRelationship
 
@@ -199,6 +201,18 @@ The relation between this StopTime and the static schedule.
 | **NO_DATA** | No data is given for this stop. It indicates that there is no realtime information available. When set NO_DATA is propagated through subsequent stops so this is the recommended way of specifying from which stop you do not have realtime information. When NO_DATA is set neither arrival nor departure should be supplied. |
 | **UNSCHEDULED** | The vehicle is operating a frequency-based trip (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 `stop_time_updates` with `schedule_relationship: UNSCHEDULED` must also set the TripDescriptor `schedule_relationship: UNSCHEDULED` <br>**Caution:** this field is still **experimental**, and subject to change. It may be formally adopted in the future.<br>.
 
+## _message_ StopTimeProperties
+
+Realtime update for certain properties defined within GTFS stop_times.txt.
+
+**Caution:** this message is still **experimental**, and subject to change. It may be formally adopted in the future.<br> 
+
+#### Fields
+
+| _**Field Name**_ | _**Type**_ | _**Required**_ | _**Cardinality**_ | _**Description**_ |
+|------------------|------------|----------------|-------------------|-------------------|
+| **assigned_stop_id** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Optional | One | Supports real-time stop assignments. Refers to a `stop_id` defined in the GTFS `stops.txt`. <br> 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`. <br> To assign a stop without providing any real-time arrival or departure predictions, populate this field and set `StopTimeUpdate.schedule_relationship = NO_DATA`. <br> If this field is populated, `StopTimeUpdate.stop_sequence` must be populated and `StopTimeUpdate.stop_id` should not be populated. Stop assignments should be reflected in other GTFS-realtime fields as well (e.g., `VehiclePosition.stop_id`). <br>**Caution:** this field is still **experimental**, and subject to change. It may be formally adopted in the future.<br>. |
+
 ## _message_ TripProperties
 
 Defines updated properties of the trip
@@ -225,7 +239,7 @@ Realtime positioning information for a given vehicle.
 | **vehicle** | [VehicleDescriptor](#message-vehicledescriptor) | Optional | One | Additional information on the vehicle that is serving this trip. Each entry should have a **unique** vehicle id. |
 | **position** | [Position](#message-position) | Optional | One | Current position of this vehicle. |
 | **current_stop_sequence** | [uint32](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Optional | One | 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. |
-| **stop_id** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Optional | One | Identifies the current stop. The value must be the same as in stops.txt in the corresponding GTFS feed. |
+| **stop_id** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Optional | One | Identifies the current stop. The value must be the same as in stops.txt in the corresponding GTFS feed. If `StopTimeProperties.assigned_stop_id` is used to assign a `stop_id`, this field should also reflect the change in `stop_id`. |
 | **current_status** | [VehicleStopStatus](#enum-vehiclestopstatus) | Optional | One | The exact status of the vehicle with respect to the current stop. Ignored if current_stop_sequence is missing. |
 | **timestamp** | [uint64](https://developers.google.com/protocol-buffers/docs/proto#scalar) | Optional | One | 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). |
 | **congestion_level** | [CongestionLevel](#enum-congestionlevel) | Optional | One |