To facilitate the testing of new features and to allow developers to add extra information to a GTFS Realtime feed, we will take advantage of the Extensions feature of Protocol Buffers. Extensions allow us to define a namespace in a Protocol Buffer message where third-party developers can define additional fields without the need to modify the original proto definition.

When a developer is interested in extending the GTFS Realtime Specification, they should contact the GTFS Realtime mailing list and we will assign them the next available extension id, picked incrementally from a list of numbers starting at 1000 and going up and documented in the Extension Registry section found below.

These assigned extension ids corresponds to the tag ids available in the "extension" namespace for each GTFS Realtime message definition. Now that the developer has an assigned extension id, they will use that id when extending any and all GTFS Realtime messages. Even if the developer only plans to extend a single message, the assigned extension id will be reserved for ALL messages.

For a developer extending the spec, instead of adding a single field like a "string" or "int32" with their extension id, the preferred model is to define a new message like "MyTripDescriptorExtension", extend the underlying GTFS Realtime message with your new message, and then put all your new fields in there. This has the nice property that you can manage fields within your extension message however you want, without needing to reserve a new extension id from the master list.

message MyTripDescriptorExtension {
  optional string some_string = 1;
  optional bool some_bool = 2;
extend transit_realtime.TripDescriptor {
  optional MyTripDescriptorExtension my_trip_descriptor = YOUR_EXTENSION_ID;

Extension Registry

Extension ID Developer Contact Details
1000 OneBusAway onebusaway-developers https://github.com/OneBusAway/onebusaway/wiki/GTFS-Realtime-Resources
1001 New York City MTA mtadeveloperresources http://mta.info/developers/
1002 Google transit-realtime-partner-support@google.com Google Maps Live Transit Updates
1003 OVapi gtfs-rt at ovapi.nl http://gtfs.ovapi.nl
1004 Metra William Ashbaugh w.l.ashbaugh@gmail.com
1005 Metro-North Railroad John Larsen
1006 realCity David Varga http://realcity.io
1007 NSW TrainLink Gregory Nicholls Group discussion