Green Line (MBTA)

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.