The Fairmount Line or Dorchester Branch is a line of the MBTA Commuter Rail system in Boston, Massachusetts, USA. Except for a short portion in Milton, it lies entirely within Boston, running southwest from South Station through the neighborhoods of Dorchester, Mattapan and Hyde Park. Weekend service began on November 29, 2014. Most trains reverse direction at the south end at Readville, but some Franklin Line trains use the Fairmount Line rather than the Northeast Corridor. On October 21, 2019, some Fairmount Line trains were extended over the Franklin Line to Foxboro as part of the Foxboro Line.