Green Line D branch

The Green Line D branch (also referred to as the Highland branch or Riverside Line) is a light rail line in Boston, Brookline, Cambridge, Newton, and Somerville, Massachusetts, operating as part of the Massachusetts Bay Transportation Authority (MBTA) Green Line. The line runs on a grade separated surface right-of-way for 9 miles (14 km) from Riverside station to Fenway station. The line merges into the C branch tunnel west of Kenmore, then follows the Boylston Street subway and Tremont Street subway to North Station. It is the longest and busiest of the four Green Line branches. As of February 2023, service operates on 8 to 9-minute headways at weekday peak hours and 8 to 11-minute headways at other times, using 13 to 19 trains (26 to 38 light rail vehicles).

Unlike the other three Green Line branches, the D branch did not originate as a streetcar line running on city streets. 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. Ownership passed to the MBTA in 1964; Tremont Street subway service was designated as the Green Line in 1965, with the Riverside Line becoming the D branch in 1967.

The line was substantially rebuilt in the mid-1970s, in 2007, and in 2018–2020. 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. Nine of the line's thirteen surface stations have been rebuilt for accessibility; the remaining four are scheduled for rebuilding beginning in 2024.