The subway system has three rapid transit lines — the Red, Orange and Blue Lines — and two streetcar/light rail lines — the Green Line and the Ashmont-Mattapan High Speed Line (considered part of the Red Line). All four colored lines meet downtown at a square configuration, and the Orange and Green Lines (which run parallel) meet directly at two stations. The Red Line has two branches in the south — Ashmont and Braintree, named after their terminal stations — and the Green Line has four branches in the west — "B" (Boston College), "C" (Cleveland Circle), "D" (Riverside) and "E" (Heath Street). The "A" Branch formerly went to Watertown, filling in the pattern, which increases from north to south, and the "E" Branch formerly continued beyond Heath Street to Arborway. The colors were assigned on August 26, 1965, and now serve as the primary identifier for the lines, after the 1964 reorganization of the MTA into the MBTA.
The Orange Line is so named because it used to run down Orange Street (now lower Washington Street), the Green Line is named because it runs adjacent to parts of the Emerald Necklace, the Blue line is named because it runs under Boston Harbor and the Red Line is named because it runs through Cambridge, Massachusetts where Harvard University (whose school color is Crimson) is located. The three rapid transit lines are incompatible in dimensions; trains of one line would have to be modified to run on another. Except between the Red Line and Ashmont-Mattapan High Speed Line, there are no track connections between lines, but all lines but the Blue Line have existing but unused connections to the national rail network, which have been used for deliveries.
Template:Merge Boston's subway was the first in the United States, and is often called "America's First Subway" by the MBTA and others. The original sections of subway, forming the Tremont Street Subway, the core of the precursor to the Green Line, opened in 1897 and 1898, and were built by the Boston Elevated Railway to take streetcars from many points off downtown streets, and would not be classified as a rapid transit system like most systems called subways. In 1901, the Main Line Elevated, the precursor to the Orange Line opened, a rapid transit line running as an elevated railway through outlying areas and using the Tremont Street Subway downtown (with the outer tracks and platforms reconfigured for Elevated trains); the Atlantic Avenue Elevated opened soon after, providing a second route downtown. This was the first elevated railway and the first rapid transit line in Boston, still coming three years before the first underground line of the New York City Subway (but long after the first elevated railway in New York).
The Washington Street Tunnel opened in 1908, giving the Elevated a shorter route through downtown and returning the Tremont Street Subway to full streetcar service. Various extensions and branches were built to the Tremont Street Subway in both directions, bypassing more surface tracks. In addition, when the Main Line El opened in 1901, many surface routes were cut back to its terminals (Dudley and Sullivan) to provide a transfer for a faster route downtown. Elevated extensions were soon built on each end, and more streetcar lines were cut back.
The next line to open was the East Boston Tunnel, a streetcar tunnel under Boston Harbor to East Boston, in 1904. This replaced a transfer between streetcars and ferries, and provided access to the other subways downtown. The tunnel was converted to rapid transit specifications in 1924, with an easy cross-platform transfer at the East Boston end.
The Cambridge Tunnel opened in 1912, connecting the downtown lines to Harvard Square in Cambridge, and was soon extended south from downtown to Dorchester as the Dorchester Tunnel. The Dorchester Extension, opening in stages from 1927, took the line further along a former New York, New Haven and Hartford Railroad branch through Dorchester, with the Ashmont-Mattapan High Speed Line continuing along the old right-of-way to Mattapan. This too resulted in cutbacks in streetcar service to its terminals.
Over the years, starting in 1922, (and possibly as the result of the alleged General Motors streetcar conspiracy), streetcar lines have been replaced with trackless trolleys coming along in 1936. By the beginning of 1953, the only remaining streetcar lines fed two tunnels — the main Tremont Street Subway network downtown and the short tunnel (now the Harvard Bus Tunnel) in Harvard Square. The Harvard routes were replaced with trackless trolleys in 1958, and are the only surviving MBTA trackless trolley routes not counting the new phase 2 Silver Line and a short non-revenue connection from the terminus of the 71 to the Watertown Carhouse. A new branch to the downtown subway opened in 1959 — the Highland Branch — using a former Boston and Albany Railroad right-of-way, and requiring many more cars than expected due to heavy ridership. The last cars to the Pleasant Street Portal ran in 1962, and it has since been covered over. The Watertown Branch hung on until 1969, two years after it was labeled as the "A" Branch, before it too was replaced by buses. The last cars to Arborway on the "E" Branch ran in 1985, and many area residents are still trying to get service extended back past Heath Street.
The old elevated railways proved to be an eyesore and required several sharp curves in Boston's twisty streets. The Atlantic Avenue Elevated was closed in 1938. The beginning of the decline of the Atlantic Avenue line was the Boston molasses disaster of 1919, which interrupted service on the line.
In the 1970s the MBTA received a boost from the BTPR areawide re-evaluation of the role of transit relative to highways. Producing a moratorium on highway construction inside Route 128, numerous transit lines were planned for expansion by the Voorhees-Skidmore, Owings and Merrill-ESL consulting team. The Charlestown Elevated, part of the Orange Line north of downtown Boston, was replaced by the Haymarket North Extension in 1975, and the Washington Street Elevated lasted until 1987, when the Southwest Corridor was opened to replace it. The closure of the Washington Street Elevated south of downtown Boston brought the end of rapid transit service to the Roxbury neighborhood. Both of these were built next to existing rail corridors.
With the 2004 closure of the Causeway Street Elevated, part of the Green Line, the only remaining elevated railways are a short portion of the Red Line at Charles/MGH and a short portion of the Green Line between Science Park and Lechmere.
The Revere Extension (now part of the Blue Line) to Wonderland opened from 1952 to 1954, mostly along the former narrow-gauge Boston, Revere Beach and Lynn Railroad right-of-way. The Braintree Extension, a branch of the Red Line to Braintree, opened in stages from 1971 to 1980, again next to an existing rail corridor. The Red Line Northwest Extension to Alewife opened in 1985, with an intermediate opening in 1984, partly along a railroad corridor and partly through a deep-bore tunnel.
These recent extensions provided not only additional subway system coverage, but also major parking structures at several of the terminal and intermediate stations, the best-known of which is Alewife, where the Route 2 freeway ends at the Red Line terminal.
On January 12, 2005, the cities of Medford and Somerville announced their intent to sue the Commonwealth of Massachusetts. The Central Artery/Tunnel Project rerouted a lot of traffic through the area, causing high levels of pollution and congestion. Though the MBTA had agreed to extend the Green Line through the two cities, there had been no progress on the extension since the deal was made in 1990. Soon after, the MBTA announced that it would build the extension.