The Mattapan High Speed Trolley goes from Mattapan to Ashmont, where riders can connect with the Red Line. While most people consider it to be part of the Red Line (it's even marked as such on transit maps), operationally it is still part of the Green Line, from which the motormen are drawn. The trolley cars also get trucked over to the Green Line's Riverside shops for any serious repairs. When the line was originally built, it had connections to the rest of Boston's substantial streetcar network at both ends, and the line's operation was based out of the Park Street Carhouse in Dorchester, located immediately south of the Red Line's Fields Corner station. The only remains of the streetcar lines that operated in Dorchester and Roxbury south of what's now the E branch of the Green Line are the MHST and a few rails poking up out of the pavement at Fields Corner.

The line uses 1940's-vintage PCC cars, which were overhauled in the late 1990's and early 2000's and have at least another 15 years of life in them. Although I have seen one report that the rails are able to support the weight of the Green Line's big articulated LRVs, no LRV has ever operated on this line. The T once planned to run them here, but dropped those plans in the late 1970's after the Boeing LRVs (the first ones on the T) proved to be a maintenance nightmare, and frequently trucking them between Mattapan and Riverside would have been a major pain in the ass.

From June 24, 2006 to December 22, 2007 the line was shut down as part of the reconstruction of Ashmont, which involved building a new trolley loop there. The other stations also received renovations at the same time. Unfortunately, the new loop did away with the cross-platform transfer that trolley riders had enjoyed since the line first opened in 1929.