GTFS+
GTFS+ is an extension to GTFS in use by the San Francisco Bay Area's Metropolitain Transportation Commission (MTC) 511.org transit trip planner. Many transit agencies in the SF Bay Area publish GTFS+.
The extension defines additional files to describe discounted fares, route and stop attributes, and to facilitate connections to real-time information.
The format is defined in File:GTFS+ Additional Files Format Ver 1.7.pdf (PDF).