MBTA nomenclature

Each is assigned a color, as follows: The Red Line has two branches, in the south, called by their terminals – Ashmont and Braintree.

The Green Line is a streetcar/light rail system with four branches to the west, labeled "B" to "E" from north to south.

Rollsigns and newer electronic signs use the following labels: Until 1965, the lines were called by their names: They were also known by various numbers, used only on maps (see below for more details), as part of an integrated system of rapid transit, streetcars and buses: After taking over operations in August 1964, the MBTA began rebranding many elements of Boston's public transportation network.

On August 26, 1965, the four rapid transit lines were assigned colored names related to their history and geography.

The Green Line branches were lettered A to E in 1967, ending the use of map numbers for the remaining rail routes.

Numbers above 202 are used by outlying routes, assigned by region as follows: The Silver Line Phase I (Washington Street) service have SL4 & SL5.

The branches of Phase II (South Boston Waterfront) are assigned labels SL1, SL2, and SL3.

As the MBTA began to take over suburban bus systems, it assigned them numbers in higher ranges.

The routes were given unused numbers from 52 to 76 in September 1982, and in 1996 the ones operating express to downtown Boston via the Mass Pike were again assigned a 5 in front.