The Ashmont–Mattapan High Speed Line, also known as the "M-Line", is a partially grade-separated light rail line which forms part of the  MBTA's Red Line rapid transit line. The line, which runs through Boston and Milton, Massachusetts, opened on August 26, 1929 as a conversion of a former commuter rail line and exclusively uses historic PCC streetcars for rolling stock. Passengers must transfer at Ashmont to access the rest of the Red Line, which uses heavy rail metro rolling stock.