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The "B" Branch, also called the Commonwealth Avenue Branch or Boston College Branch, is a branch of the Massachusetts Bay Transportation Authority Green Line in the Boston, Massachusetts area, along which light rail vehicles run on a surface right-of-way down the middle of Commonwealth Avenue. After going underground via the Blandford Street Incline, the tracks merge with the "C" and "D" Branches into Kenmore station. From there the Boylston Street Subway and Tremont Street Subway carry "B" cars to downtown Boston, with regular service turning around at Government Center as of November 2005.

In 1896, tracks were laid on Commonwealth Avenue from Chestnut Hill Avenue west to the Newton town line at Lake Street, and around the same time the Commonwealth Avenue Street Railway opened, extending the tracks through Newton to Norumbega Park; this later became part of the Middlesex and Boston Street Railway. Trains between Lake Street and downtown Boston used tracks on Beacon Street, now part of the "C" Branch. From Kenmore Square they continued east on Beacon Street, then turned south on Massachusetts Avenue and east on Boylston Street to Park Square. In 1900 tracks were installed on the rest of Commonwealth Avenue, from Chestnut Hill Avenue east to existing tracks at Packard's Corner, later part of the "A" Branch. This enabled trains to use Commonwealth Avenue between Lake Street and Kenmore Square. In 1909, the tracks were electrified.^[1]

The Tremont Street Subway opened on September 1, 1897, and then or soon after the Commonwealth Avenue line was rerouted to turn around at Park Street via the Boylston Street Incline at the Public Gardens. The Boylston Street Subway opened on October 3, 1914, extending the underground portion to the Kenmore Incline just east of Kenmore Square. On October 23, 1932 the Blandford Street Incline opened along with the underground Kenmore station, giving the line its present configuration.

Until the 1920s, a streetcar line that has since become the 65 bus ran along Commonwealth Avenue from Boston College east to Chestnut Hill Avenue, then south on Chestnut Hill Avenue and east on Beacon Street from Cleveland Circle to Washington Square, turning southeast to Brookline Village. After this was replaced by bus service in the 1920s (along with a rerouting), no revenue service used the tracks on Chestnut Hill Avenue, but they have remained for "B" Branch trains to access the Reservoir Carhouse at Cleveland Circle on the "C" Branch.

From 1942 to 1967, the route was known by the map number of 62. Since then it has been the "B" Branch.

A turnback loop at Boston University Field was present from 1915 to January 14, 1962, and was used for service to special events.

On February 7, 1931, the Commonwealth Avenue service was extended east through downtown to loop at Lechmere rather than Park Street. Since then, the following changes in east terminal have been made:

• November 20, 1961: cut back to Park Street
• November 18, 1964: extended Monday-Saturday to Government Center
• January 1967: cut back to Park Street all times due to problems at the Government Center loop
• March 25, 1967: extended to North Station
• June 17, 1967: cut back to Government Center
• September 9, 1967: extended to North Station
• September 14, 1967: cut back to Government Center due to problems at the North Station loop
• March 21, 1970: extended to Haymarket, looped empty at North Station
• December 26, 1970: extended to North Station
• March 25, 1974: extended to Lechmere
• January 1, 1977: cut back to North Station weekdays only
• March 19, 1977: extended to Lechmere all times
• June 18, 1977: cut back to Government Center all times
• September 9, 1977: extended to Lechmere all times
• September 9, 1978: cut back to Government Center nights and Sundays
• March 24, 1979: cut back to Government Center middays and Saturdays, with only rush hour trips continuing to Lechmere
• March 21, 1980: cut back to Park Street all times
• April 4, 1981: extended to North Station (PCC cars looped at Government Center)
• July 30, 1983: cut back to Government Center

Four stops were temporarily closed for 6-8 months starting April 20, 2005 as part of a stop elimination pilot program to speed up service. At the end of the test period, they were closed permanently. The discontinued stops were very close to more popular intersections, and almost 73% of the 1142 riders surveyed wanted those four stops gone.^[2] The B line service is slower than the C branch because of additional stops, and both are slower then the D branch, which has a dedicated right-of-way.

Station	Location	Time to Park Street	Transfers and notes
Blandford Street	Commonwealth Avenue at Sherborn Street, Boston	16 minutes	no MBTA parking
Boston University East	Commonwealth Avenue at Granby Street, Boston	17 minutes	the first station serving Boston University
Boston University Central	Commonwealth Avenue at Marsh Chapel, Boston	18 minutes	the second station serving Boston University
Boston University West	Commonwealth Avenue at Amory Street, Boston	19 minutes	the third station serving Boston University
St. Paul Street	Commonwealth Avenue at Buick Street, Boston	20 minutes	57 Bus to Watertown Square
Pleasant Street	Commonwealth Avenue at Pleasant Street, Boston	22 minutes	no MBTA parking
Babcock Street	Commonwealth Avenue at Babcock Street, Boston	24 minutes	no MBTA parking
Packard's Corner	Packard's Corner at Brighton Avenue, Boston	26 minutes	the "A" Branch formerly diverged here
Harvard Avenue	Commonwealth Avenue at Harvard Avenue, Boston	28 minutes	66 Bus to Harvard Square
Griggs Street	Commonwealth Avenue at Griggs Street, Boston	30 minutes	no MBTA parking
Allston Street	Commonwealth Avenue at Allston Street, Boston	32 minutes	no MBTA parking
Warren Street	Commonwealth Avenue at Warren Street, Boston	34 minutes	no MBTA parking
Washington Street	Commonwealth Avenue at Washington Street, Boston	36 minutes	65 Bus to Kenmore Square
Sutherland Street	Commonwealth Avenue at Sutherland Street, Boston	38 minutes	no MBTA parking
Chiswick Road	Commonwealth Avenue at Chiswick Road, Boston	40 minutes	no MBTA parking
Chestnut Hill Avenue	Commonwealth Avenue at Chestnut Hill Avenue, Boston	41 minutes	within easy walking distance of the C branch's Cleveland Circle station, and the D branch's Reservoir station
South Street	Commonwealth Avenue at South Street, Boston	42 minutes	no MBTA parking
Boston College	Commonwealth Avenue at Lake Street, Boston, serving Boston College	43 minutes	formerly Lake Street

• Green Line schematic

1. ^ http://www.bahistory.org/Clevcirclehist.html
2. ^ T drops 4 Green Line stops after results of rider survey, The Boston Globe March 16, 2005

• Changes to Transit Service in the MBTA district (PDF)
• U.S. Urban Rail Transit Lines Opened From 1980 (PDF)