from Google's "What is GTFS-realtime?" guide:
GTFS-realtime is a feed specification that allows public transportation agencies to provide realtime updates about their fleet to application developers. It is an extension to GTFS (General Transit Feed Specification), an open data format for public transportation schedules and associated geographic information. GTFS-realtime was designed around ease of implementation, good GTFS interoperability and a focus on passenger information.
The specification was designed through a partnership of the initial Live Transit Updates partner agencies, a number of transit developers and Google. The specification is published under the Apache 2.0 License.
Components of GTFS-realtime
This is a stub section that needs to be developed. See "feed types".
How to provide GTFS-realtime
This is a stub section that needs to be developed.
Applications that consume GTFS-realtime data
List is not comprehensive