The Ashmont–Mattapan High-Speed Line, commonly referred to as the Mattapan Trolley, 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. It exclusively uses PCC streetcars, supported by a maintenance team of blacksmiths and metalworkers who make parts for the historic rolling stock. Passengers must transfer at Ashmont to access the rest of the Red Line, which uses heavy rail metro rolling stock. The trolley's route is used only by streetcars and has just two public grade crossings. All stations have low platforms, but all except Valley Road have been retrofitted with wheelchair lifts or wooden ramps for accessibility. Unlike most heritage streetcar lines, it is an integral part of the modern MBTA transit system rather than a tourist attraction. History Commuter rail lines The Dorchester and Milton Branch Railroad opened in December 1847 from Neponset on the Old Colony Railroad main line to Mattapan station in Dorchester via Milton Mills (later Milton Lower Mills, then simply Milton). The line was immediately leased by the Old Colony as its Milton branch. The Old Colony built the Shawmut Branch Railroad from Harrison Square on the main line to Milton Lower Mills via Peabody Square in 1872. Most Mattapan passenger service switched to use the new branch east of Milton, as it ran through dense urban neighborhoods rather than swamps. The Old Colony Railroad and its branches were acquired by the New York, New Haven and Hartford Railroad in 1893. When the Boston Elevated Railway (BERy) was first constructing its rapid transit Cambridge–Dorchester line in the early 1910s, plans called for the line to be extended south from Andrew to Codman Square via Edward Everett Square, Columbia Square, and Mount Bowdoin. The route would have paralleled the New Haven's Shawmut branch and Midland Division. By the end of that decade, however, passenger traffic on both New Haven-owned lines had been decimated by the BERy's network of electrified streetcar lines, which connected to rapid transit trains at , , , and . Around 1920, BERy reached an agreement with the New Haven and the Boston Transit Commission to pursue the Dorchester Circuit Plan. Under that plan, a bidirectional rapid transit loop would run south from Andrew along the Old Colony main line,