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 from Riverside station to Fenway station. The line merges into the C branch tunnel west of , then follows the Boylston Street subway and Tremont Street subway to . It is the longest and busiest of the four Green Line branches. , service operates on 7-minute headways at weekday peak hours and 8 to 11-minute headways at other times, using 11 to 17 trains (22 to 34 LRVs). 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 , Government Center, North Station, and a number of times; it was extended to 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 in 2023–24. History Commuter rail What is now the surface section of the D branch was formerly the Highland branch of the Boston and Albany Railroad (B&A). The portion between Brookline Junction and was built by B&A predecessor Boston and Worcester Railroad in 1848. The Charles River Branch Railroad extended the line to Newton Upper Falls in 1852, and later to Woonsocket, Rhode Island. 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. A short extension from Cook Street to on the B&A mainline opened in 1886. The B&A instituted bidirectional "Newton Circuit" loop service via the Highland branch and mainline, while the NY&NE continued to use the branch