The line begins at Cleveland Circle in the Brighton neighborhood of Boston and runs on the surface through Brookline along the median of Beacon Street.

Reentering Boston, the line goes underground through the Saint Mary's Street incline and joins the B and D branches at Kenmore.

[9] On January 9, 1930, the BERy began running Washington Square-bound streetcars express from Kenmore to Kent Street in the afternoon peak due to crowding.

[15]: 202  The first use of a three-car train of PCC cars was on July 13, 1946 for a baseball extra; they entered regular service on September 16, 1946.

Trains used a temporary loop at Reservoir and non-revenue tracks on Chestnut Hill Avenue to cross between the two lines.

[8] From March 20 to June 25, 1976, C branch night service looped at Kenmore to allow electrical work in the central subway.

[8] The C branch was cut to Government Center on October 24, 2021, as part of changes in preparation for the opening of the Green Line Extension later in the year.

[17] C branch service was replaced by buses from July 11 to 22, 2022, to allow for trackwork and installation of train protection system equipment.

In the early 2000s, the MBTA modified key surface stops with raised platforms as part of the Light Rail Accessibility Program.

[19][20] Four surface stops – Saint Mary's Street, Coolidge Corner, Washington Square, and Cleveland Circle – were modified with raised platforms in 2002–03.

[23] In May 2024, the Federal Transit Administration awarded the MBTA $67 million to construct accessible platforms at the 14 B and C branch stops.

The signals on Beacon Street in Brookline could in theory be prioritized to make the Green Line run faster.

[31] As of 2011[update], the Town of Brookline was considering formally asking the MBTA to cooperate in setting up traffic signal prioritization to speed up Green Line trains on Beacon Street.

[32] In Mayor Marty Walsh's "Go Boston 2030" plan, prioritizing traffic signals on Beacon Street was a proposed idea.