This is very possible [1]. I use NextBus for the real time bus mapping even though the MBTA offers it because I've found NextBus to be much more accurate and stable.

I'm a little nervous because I might be violating their terms of service in using their feed without being an official customer but it's been running for a couple of years now without issue.

[1] http://webservices.nextbus.com/service/publicXMLFeed?command...