thumb|300px|Boston-area streetcar lines remaining in 1940 (in green), plotted against a map of the BERy's subway and elevated lines (in purple). The shade of green for each line denotes how long the line lasted after this; the lightest-green lines were abandoned in 1945 or earlier, the second-lightest lines were abandoned from 1946 to 1950, the second-darkest lines were abandoned from 1951 to 1969, and the darkest-green lines still existed in 1969.
As with many large cities, a large number of Boston-area streetcar lines once existed, and many continued operating into the 1950s. However, only a few now remain, namely the four branches of the Green Line and the Mattapan Line, with only one (the Green Line E branch) running regular service on an undivided street.
History
The first streetcar line in the Boston area was a horse-drawn line from Central Square, Cambridge to Bowdoin Square, Boston opened by the Cambridge Railroad on March 26, 1856. Over the following decade a large number of horsecar lines were built by different companies, including the Metropolitan Railroad, Middlesex Railroad, and South Boston Railroad; these companies competed with each other while also sharing tracks in many locations. By the mid-1860s horsecar lines reached to Lynn, Arlington, Watertown, Newton, West Roxbury, and Milton. In 1887 the various Boston-area horsecar companies (except for the Lynn and Boston Railroad) were all consolidated into the West End Street Railway.
In 1889 the West End Street Railway experimented with electric power for its streetcars; the results were so promising that it abandoned a cable car project already under construction. Several lines were electrified in 1889 and by 1895 almost the entire system had been electrified. The last horsecar line was abandoned in 1900.
In 1897 the recently formed Boston Elevated Railway (BERy) took over the West End Street Railway in order to make the streetcar lines part of its planned rapid transit system. In 1897 the Tremont Street Subway opened and many streetcar routes that had previously used surface tracks in downtown Boston were rerouted into the subway. Over the following decades the opening elevated and underground rapid transit lines (which became today's Orange Line and Red Line), as well as extensions of the Tremont Street subway (which became the Green Line), allowed progressively more streetcar lines to be removed from the congested streets downtown and rerouted to rapid transit stations further out. Passengers could transfer for free between streetcars and rapid transit lines to complete their journeys to or from downtown. Additionally, the numbers were only kept the same on and after the 1942 revision of the map; before that they were changed with each new version. A few routes were renumbered around 1967, but most routes have kept their original numbers even through conversions from streetcar to trackless trolley to bus.
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|June 14, 1956
|bgcolor=dfffdf| Egleston–subway
|Cut back to Lenox Street
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|rowspan=2|December 16, 1955
|<cite id="40"></cite>40 Arborway–Egleston
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|<cite id="100"></cite> Elm Street–Sullivan
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|November 18, 1955
|<cite id="79"></cite>79 Arlington Heights–Harvard
|Replacement bus route renumbered ca. 1967
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|September 9, 1955
|<cite id="29"></cite> Mattapan–Egleston
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|rowspan=2|December 4, 1953
|bgcolor=dfffdf|<cite id="9"></cite> City Point–subway
|Short-turn of 116/117<br>Trackless trolley until ca. 1954 (Woodlawn–Wood Island)
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|<cite id="116"></cite> Revere Beach Loop–Maverick via Revere Street
|Had run into the subway via the Mystic River Bridge and North Station until 1934<br>Trackless trolley until September 8, 1961 (Wonderland–Wood Island)
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|<cite id="117"></cite> Revere Beach Loop–Maverick via Beach Street
|Had run into the subway via the Mystic River Bridge and North Station until 1934<br>Trackless trolley until September 8, 1961 (Wonderland–Wood Island)
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|<cite id="118"></cite>118 Revere carhouse–Maverick via Ocean Avenue and Bennington Street
|Trackless trolley until June 18, 1955 (Wonderland–Orient Heights)
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|<cite id="120"></cite> Gladstone–Maverick
|Short-turn of 118<br>Trackless trolley until September 8, 1961 (Orient Heights–Maverick)
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|<cite id="121"></cite> Eagle Street–Maverick via Lexington Street
|Trackless trolley until March 30, 1961 (Wood Island–Maverick)
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|September 28, 1951
|<cite id="36"></cite> Charles River Loop–Arborway
|Trackless trolley until September 5, 1958
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|June 16, 1950
|<cite id="70"></cite> Watertown–Central
|Trackless trolley until March 30, 1963
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|June 12, 1950
|colspan=2|Meridian Street Bridge closes for construction; 114 cut back to south end of bridge, 116 and 117 rerouted via Chelsea Street Bridge until November 9
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|September 16, 1949
|<cite id="42"></cite> Egleston–Dudley
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|July 1, 1949
|bgcolor=dfffdf|<cite id="93"></cite> Sullivan–subway via Bunker Hill Street
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|June 7, 1949
|<cite id="41"></cite> Jamaica Plain–Dudley
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|April 22, 1949
|<cite id="123"></cite>123 Jefferies Point–Maverick
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|February 11, 1949
|<cite id="17"></cite> Fields Corner–Andrew via Meeting House Hill
|Trackless trolley until April 5, 1962
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|January 28, 1949
|<cite id="44"></cite> Seaver–Dudley
|Trackless trolley until March 30, 1961
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|rowspan=3|January 7, 1949
|<cite id="19"></cite> Fields Corner–Dudley via Geneva Avenue
|Trackless trolley until April 5, 1962
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|<cite id="22"></cite> Ashmont-Dudley via Talbot Avenue
|Trackless trolley until April 5, 1962
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|<cite id="23"></cite> Ashmont-Dudley via Washington Street, Dorchester
|Trackless trolley until April 6, 1962
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|rowspan=3|December 24, 1948
|<cite id="15"></cite> Uphams Corner–Dudley
|Trackless trolley until April 5, 1962 (Kane Square–Dudley)
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|<cite id="45"></cite> Grove Hall–Dudley via Blue Hill Avenue
|Trackless trolley until April 5, 1962
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|bgcolor=dfffdf|Dudley–subway late night service
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|bgcolor=dfffdf|Brookline Village–subway via Huntington Ave
|Remained in use for late night service until December 24, 1948
|Had run into the subway via the Mystic River Bridge and North Station until 1934
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|June 10, 1936
|colspan=2|EMSR Chelsea division routes (111–113 and 116–119) acquired by BERy with several rerouted to East Boston; some of these had run into the subway via the Mystic River Bridge and North Station until 1934
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|June 10, 1934
|bgcolor=dfffdf|<cite id="58"></cite>58 Cypress Street–subway via Huntington Avenue
|Rush-hour service between the subway and Brookline Village continued until 1938
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|January 13, 1934
|bgcolor=dfffdf colspan=2|Mystic River Bridge (between Chelsea and Charlestown) closes for construction; EMSR routes from Woodlawn, Beachmont, Revere, Lynn and Salem cut back from the subway to Chelsea
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|September 16, 1933
| Linden–Everett via Broadway
|Cut back to Everett carhouse (at Broadway and Cameron Street)
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|November 22, 1929
|<cite id="26"></cite> Norfolk Street–Dudley
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|July 18, 1925
|<cite id="81"></cite>81 Arlington Center–Clarendon Hill
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|July 9, 1922
|bgcolor=dfffdf colspan=2|Last day of surface lines (77, 87, 88) entering the subway via
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|}
MTA streetcar routes as of 1953
thumb|300px|A map of the MTA's transit network in 1952, with streetcars indicated by route numbers in circles, buses by route numbers in rectangles, and trackless trolleys by route numbers in triangles
As of early 1953 the Metropolitan Transit Authority operated the following streetcar routes. All lines were connected via trackage to the Tremont Street subway (Green Line), but only the 9, 39, 43, 57, 61, 62 and 69 actually operated in the subway.