Short for the
MBTA, which is itself an abbreviation for
Massachusetts Bay Transportation Authority, the T usually refers to the
subway system in and around
Boston, and sometimes to the area's
commuter rail system as well. Origin is probably the signs outside stations, which consist of a black T on a round white sign.
The Green Line of the T is the oldest subway system in the United States, operating since 1897.
Improvements to the T are part of The Big Dig, an inevitable result of the fact that the T runs right through the same area that they're trying to construct an underground highway in.