MBTA subway

The rapid transit lines consist of 3 heavy rail lines, 2 light rail lines, and an underground bus rapid transit line.

The traditional heavy rail lines include the Blue Line, which is a former trolley line running from Revere to downtown Boston; the Orange Line, which was converted from an elevated line running from Roxbury to Malden; and the Red Line, running from Cambridge to either Ashmont or Braintree.

The light rail lines include 4 Green Line branches which terminate in Brighton, Brookline, Newton, and Roxbury, and the solely Red Line-linked Mattapan High Speed Line.

Three branches of the Silver Line, the SL1, SL2, and SL3, operate in tunnels for part of their length, with direct transfers at South Station. These three lines use the higher subway fare, while SL4 and SL5 follow bus fare rules.

An unofficial schematic map of the rapid transit system (plus non-BRT key bus routes) from 2013. The official MBTA map is an altered version of this map, which won a redesign contest in 2014.

All four subway lines cross downtown, forming a quadrilateral configuration, and the Orange and Green Lines (which run approximately parallel in that district) also connect directly at two stations just north of downtown. The Red Line and Blue Line are the only pair of subway lines which do not have a direct transfer connection to each other. Because the various subway lines do not consistently run in any given compass direction, it is customary to refer to line directions as "inbound" or "outbound". Inbound trains travel towards the four downtown transfer stations–Park Street, State Street, Government Center and Downtown Crossing–and outbound trains travel away from these hub stations.[5]

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 north-to-south letter assignment pattern, and the "E" Branch formerly continued beyond Heath Street to Arborway.

The Red Line has two branches in the south—Ashmont and Braintree, named after their terminal stations.

Originally, transit lines in the region only used geographic names; though numbering was added to public maps in 1936. The three heavy rail lines were assigned numbers 1, 2, and 3; what is now the Green Line was assigned different numbers for each branch. However, riders generally continued to use the geographic names.[6] Colors were assigned on August 26, 1965 as part of a wider modernization under design standards developed by Cambridge Seven Associates, and have served as the primary identifier for the lines since then.[7] The numbers for the heavy rail lines and the Mattapan Line were retained in public information until 1966.[6] In 1967, the then-current five branches of the Green Line were lettered A through E.[6]

Cambridge Seven originally intended to use red, yellow, green, and blue for the four lines. However, yellow proved unsuitable, since some patrons would have difficulty reading yellow text on a white background; orange was substituted, and yellow eventually was used for the MBTA Bus service's visibility markings and signage.[8] When sketching design concepts, Peter Chermayeff labeled the subway-surface light rail routes as the Green Line because they run adjacent to parts of the Emerald Necklace park system. The East Boston Tunnel became the Blue Line because it runs under Boston Harbor, and the Cambridge-Dorchester Tunnel became the Red Line because its northernmost terminus was then at Harvard University, whose school color is crimson. According to Chermayeff, the Main Line El "ended up being orange for no particular reason beyond color balance."[9] The MBTA and transit historians later claimed that orange came from Orange Street, an early name for the street that ran southwards down the Boston Neck to connect the Shawmut Peninsula to the mainland, for what is now part of Washington Street.[10][11][8]

When the switch to colored names was made, the MBTA planned to color bus rollsigns and bus stop signs to match the destination station of the route.[12] However, that scheme was never implemented.

The types of track used on various parts of the MBTA system, and the number of stations on each type of track

The MBTA rapid transit system consists of 133 stations, with another 7 in construction as part of the Green Line Extension project in Somerville.

The MBTA is in the process of replacing its entire fleet of Red Line and Orange Line cars, which are some 30 years old, by 2023. The Blue Line cars were replaced in 2008. The Green Line has a variety of vehicles, some dating back to 1986, with the latest batch delivered in 2019.

The four transit lines all use standard rail gauge, but are otherwise incompatible. Blue Line trains can run on Orange Line tracks, but can't be used for revenue service because the two lines have different platform heights. Orange Line cars are too long to negotiate the tight turns of the Blue Line.[13] However, this limited compatibility has been taken advantage of. When new Blue Line cars were delivered from Siemens Transportation in 2007, they were tested on the Orange Line after hours, before acceptance for revenue service on the Blue Line. The Red Line is incompatible with the Orange and Blue Lines, as its cars are wider and use yet another platform height; having a train on the Blue or Orange lines run on that line would result in a relatively wide gap between the platform edge and the train. The Green Line is incompatible with the other subway lines due to having a different propulsion system.[13]

There are no direct track connections between lines, except between the Red Line and Ashmont-Mattapan High Speed Line, but all except the Blue Line have little-used connections to the national rail network, which have been used for deliveries of railcars and supplies.[14]