The original Tremont Street Subway south of Boylston station has been closed since 1962, when the last streetcar line feeding into it was replaced by bus service, and Pleasant Street Portal at its southern end has been covered over.
Reuse of part of the tunnel for the Silver Line Phase III was briefly considered, but the narrow bore was found too small for the Silver Line buses which (unlike trolleys) are not fixed to their guideway.
It was, however, included in the destination signs on the Boeing-Vertol LRVs ordered in the mid-1970s, when reopening service to Watertown was under consideration.
The A line tracks remained in non-revenue service to access maintenance facilities at Watertown until 1994.
As of 2017, regular "C" service turns around at North Station.
The "D," "Riverside" or "Highland" Branch is the southernmost of the three lines that separate west of Kenmore.
The Lechmere Viaduct originally connected to the Central Subway via the Causeway Street Elevated, a half-mile-long structure running in front of North Station and the Boston Garden sports complex.As of 2017, regular "B" service turns around at Park Street.The "C", "Cleveland Circle" or "Beacon Street" Branch is the middle one of the three branches heading west from Kenmore, and the straightest, running down the middle of Beacon Street through Brookline to Cleveland Circle.No branches had used the Canal Street Portal except as a terminal since 1949 with the 93 or the Pleasant Street Portal since 1961 with the 43, and a shuttle until 1962.All trains stop at Park Street, Boylston, Arlington, and Copley. On the eastern end, only "C" and "E" trains go past Government Center to Haymarket and North Station; the only train that services Science Park and Lechmere is the "E" Branch (although the "D" Branch will service these stations once the Medford extension is built).It is the longest branch, ending in Newton at Riverside.