Google introduced Live Transit powered by the GTFS-realtime feed format that adds realtime public transport information to Google Maps and Google Maps for mobile is now available on Google Code.
GTFS-realtime allows public transport agencies to provide realtime updates about their fleets. If you're developing a trip planner or similar application, you can process these feeds and keep your users up-to-date with realtime information. "The specification currently includes three types of realtime updates: Trip Updates, Service Alerts and Vehicle Position updates. Each type of update has to be provided in a separate feed, and can be used independently," informs Vladimir Rychev, Software Engineer.
"GTFS-realtime is an extension to GTFS, the General Transit Feed Specification, published by Google in 2006. Nowadays, GTFS is a very commonly used feed format that public transport agencies use to (publicly) provide their transport information. As opposed to GTFS feeds, GTFS-realtime feeds contain very dynamic information. This means that they have to be updated frequently and applications that use them have to fetch them frequently as well. This requires a significant infrastructure from the transport agency's side, but it results in a continuously updated description of the current situation."
"To encode realtime updates, Protocol Buffers are used. Protocol Buffer data structures can be processed very efficiently, resulting in low processing times compared to other popular data encapsulation standards. Because Protocol Buffers are compressed, they also use communication bandwidth efficiently. Protocol Buffers are very easy to work with, and there are libraries available for many programming languages," notes Rychev.