If you’re new to the General Transit Feed Specification (GTFS) format, gtfs.org is a great place to get started. We provide schedule data in GTFS, and our real-time data reflects the concepts and keys in GTFS.
Google's GTFS-realtime documentation provides an introduction to the GTFS-realtime format, which you can use to retrieve real-time service information for the whole agency.
For more Massachusetts data
The MassDOT Developers page includes data from other Massachusetts transit agencies, as well as highway, airport, and RMV data.