The Green Line is a semi-metro system (form of light rail) run by the Massachusetts Bay Transportation Authority (MBTA) in the Boston, Massachusetts, metropolitan area. It is the oldest MBTA subway line, and with tunnel sections dating from 1897, the oldest subway in North America. It runs underground through downtown Boston, and on the surface into inner suburbs via six branches on radial boulevards and grade-separated alignments. With an average daily weekday ridership of 137,700 in 2019, it is the third most heavily used light rail system in the country. The line was assigned the green color in 1967 during a systemwide rebranding because several branches pass through sections of the Emerald Necklace of Boston.
The four branches are the remnants of a large streetcar system, which began in 1856 with the Cambridge Horse Railroad and was consolidated into the Boston Elevated Railway several decades later. The branches all travel downtown through the Tremont Street subway, the oldest subway tunnel in North America. The Tremont Street subway opened its first section on September 1, 1897, to take streetcars off overcrowded downtown streets; it was extended five times over the next five decades. The streetcar system peaked in size around 1930 and was gradually replaced with trackless trolleys and buses, with cuts as late as 1985. The new D branch opened on a converted commuter rail line in 1959. The Green Line Extension project extended two branches into Somerville and Medford in 2022.
Route description
The Green Line's core is the central subway, a group of tunnels which run through downtown Boston. The Tremont Street subway runs roughly north–south through downtown, with stations at , , , , and – all with connections to other lines of the MBTA subway system. The Boylston Street subway runs roughly east–west through the Back Bay neighborhood, with stations at , , , and ; it connects to the Tremont Street subway at Boylston. The Huntington Avenue subway diverges from the Boylston Street subway at a flat junction west of Copley, running southwest with stations at and . A branch of the Tremont Street subway, disused since 1962, runs south from Boylston to the former Pleasant Street incline.
The Green Line has four western surface branches, each with a letter designation: The B branch runs west in a tunnel from Kenmore, surfacing in the median of Commonwealth Avenue at . It runs on Commonwealth Avenue through Boston University, Allston, and Brighton to Boston College station. The C branch runs southwest in a tunnel from Kenmore, surfacing in the median of Beacon Street at . It runs on Beacon Street through Brookline to Cleveland Circle station. The D branch diverges from the C branch tunnel southwest of Kenmore, surfacing at Fenway station. It runs though Brookline and Newton along the grade-separated Highland branch (a former Boston and Albany Railroad commuter rail line) to Riverside station. The E branch runs southwest through the Huntington Avenue subway from Copley, surfacing in the median of Huntington Avenue at Northeastern University station. It runs along Huntington Avenue and South Huntington Avenue to , with the outer (west of ) in mixed traffic rather than a dedicated median. Until 1985, the line continued south through Jamaica Plain to .
B and C branch service terminates at Government Center, while D and E branch service continues north from downtown Boston. North of North Station, the line inclines up to the Lechmere Viaduct, which crosses the Charles River with an elevated station at . The elevated Lechmere station is located on the Green Line Extension viaduct, which continues north to a flying junction with two branches: The Union Square Branch runs west to Union Square station via the Fitchburg Line right-of-way. It is operated as part of the D branch. The Medford Branch runs northwest to Medford/Tufts station via the Lowell Line right-of-way. It is operated as part of the E branch.