The Red Line is a rapid transit line operated by the Massachusetts Bay Transportation Authority (MBTA) as part of the MBTA subway system. The line runs south and east underground from Alewife station in North Cambridge through Somerville and Cambridge, surfacing to cross the Longfellow Bridge then returning to tunnels under Downtown Boston. It continues underground through South Boston, splitting into two branches on the surface at JFK/UMass station. The Ashmont branch runs southwest through Dorchester to Ashmont station, where the connecting light rail Ashmont–Mattapan High Speed Line continues to Mattapan station. The Braintree branch runs southeast through Quincy and Braintree to Braintree station.

The Massachusetts Bay Transportation Authority is the public agency responsible for operating most public transportation services in Greater Boston, Massachusetts. The MBTA transit network includes the MBTA subway with three metro lines, two light rail lines, and a five-line bus rapid transit system ; MBTA bus local and express service; the twelve-line MBTA Commuter Rail system, and several ferry routes. In 2023, the system had a ridership of 239,981,700, or about 755,500 per weekday as of the fourth quarter of 2023, of which the rapid transit lines averaged 267,100 and the light rail lines 80,300, making it the fourth-busiest rapid transit system and the third-busiest light rail system in the United States. As of the fourth quarter of 2023, average weekday ridership of the commuter rail system was 97,100, making it the fifth-busiest commuter rail system in the U.S.