[aftbit]

I just tried to sign up and got directed here: https://511.org/open-data/transit

It looks like the bulk data feeds are available but the APIs are not. I am particularly interested in:

    Real-time Vehicle Monitoring (SIRI)
    Real-time Vehicle Monitoring API provides information about current location and expected activities of a vehicle in XML and JSON formats.

    Endpoint: http://api.511.org/transit/TBD
    Allowable parameters: api_key (mandatory) and TBD

Also, looks like there's an error in "GTFS-Realtime Vehicle Positions" description - it claims:

    GTFS-Realtime Trip Updates provides real-time updates on the vehicle positions for an agency in the Protocol Buffer format.