The Boston and Albany Railroad Highland branch, built in segments from 1848 to 1886, operated as a commuter rail line until its 1958 closure.

It was converted to a streetcar rapid transit line by the Metropolitan Transportation Authority and reopened on July 4, 1959.

The downtown terminal was shifted between Park Street, Government Center, North Station, and Lechmere a number of times; it was extended to Union Square in September 2022 as part of the Green Line Extension project.

[5]: 339  The Charles River Branch Railroad extended the line to Newton Upper Falls in 1852, and later to Woonsocket, Rhode Island.

[5]: 352  The B&A purchased the line between Brookline Village and Cook Street (in Newton Highlands) from the New York and New England Railroad (NY&NE) in 1883.

That proposal called for trains to reach downtown Boston using the then-under-construction Boylston Street subway, which was built for streetcars but sized for rapid transit conversion, with a connection near Governor Square.

[7]: 17  In May 1924, the state legislature directed the Metropolitan District Commission to plan an expanded rapid transit system in Boston.

[8] The plan, released in 1926, called for the existing streetcar subway to be converted into two rapid transit lines.

A number of potential future extensions were listed; among them was conversion of most of the Highland branch to rapid transit, linking with a Huntington Avenue subway at Brookline Village.

[9] In 1943, the state legislature appointed a commission headed by Arthur W. Coolidge to plan for the future of transit in the Boston region.

In 1945, a preliminary report from the Coolidge Commission recommended nine suburban rapid transit extensions – most similar to the 1926 plan – along existing railroad lines.

[10][11] The Highland branch was to be converted to a rapid transit line, using low-floor trains that could operate in the existing streetcar subway.

[14] The project was approved by the MTA Advisory Board on October 1, 1957, and the state Department of Public Utilities on December 6.

[7]: 4  The work was done at relatively low cost by reusing the railroad infrastructure, including bridges and tracks; the only new right-of-way was a 1,100-foot (340 m) tunnel connecting Fenway station with the existing Beacon Street Line subway.

[19][2][22] (On March 5, 1974, the downtown terminal had been cut back from Lechmere to North Station to accommodate the longer running times associated with this rerouting.

On December 15, 1977, all D branch trains running with LRVs began using the newly rebuilt Canal Street surface terminal at North Station; the small number of PCCs continued to loop at Government Center.

It was cut back to Government Center on June 19, 2004, to accommodate the demolition of the Causeway Street elevated and the opening of the new underground North Station.

Green Line service to Lechmere resumed on November 12, 2005, though the D branch continued to terminate at Government Center.

[25][26] The National Transportation Safety Board investigation concluded that the operator likely had an episode of micro-sleep caused by sleep apnea.

[2] Work from 2018 to 2021 included a full modern signal system for the line between Beaconsfield and Riverside plus partial track replacements.

[30] Three nine-day closures of the whole D branch took place between September 24 and October 28, 2022, to allow for work including installation of a train protection system and track replacement between Eliot and Riverside.

[31][32] Riverside station was rebuilt in the late 1990s, with raised platforms to provide accessible boarding on the Type 8 light rail vehicles then being built.

[39] Design work for the remaining four non-accessible stops (Beaconsfield, Chestnut Hill, Eliot, and Waban) was completed in late 2022.

[46] The D branch was temporarily extended to North Station on October 24, 2021, as part of changes in preparation for the opening of the extension.

[2] All service was cut back to Government Center from August 22 to September 18, 2022, to allow for final integration of the Medford Branch and other work.

[2] The Union Square Branch was closed from September 18 to October 12, 2023, during repairs to Squires Bridge, which carries the McGrath Highway over the tracks.