The Green Line D branch 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.

Brookline is a town in Norfolk County, Massachusetts, United States, and part of the Boston metropolitan area. Brookline borders six of Boston's neighborhoods: Brighton, Allston, Fenway–Kenmore, Mission Hill, Jamaica Plain, and West Roxbury. The city of Newton lies to the west of Brookline. Brookline was first settled in 1638 as a hamlet in Boston, known as Muddy River; it was incorporated as a separate town in 1705.