I'm not sure, but my first guess is Metropolitan Bus Transit Authority.
MBTA, when referring to the transportation agency in Boston, stands for Massachusetts Bay Transportation Authority. It is sometimes referred to simply as "The T".
Airport - MBTA station - was created in 1952.
Beachmont - MBTA station - was created in 1954.
Ruggles - MBTA station - was created in 1987.
Reading - MBTA station - was created in 1870.
Cohasset - MBTA station - was created in 2007.
Lechmere - MBTA station - was created in 1922.
Blue Line - MBTA - was created in 1924.
Prudential - MBTA station - was created in 1941.
Wakefield - MBTA station - was created in 1889.
Chelsea - MBTA station - was created in 1985.
Orange Line - MBTA - was created in 1901.