The Orange 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 on the surface from Oak Grove station in Malden, Massachusetts through Malden and Medford, paralleling the Haverhill Line, then crosses the Mystic River on a bridge into Somerville, then into Charlestown. It passes under the Charles River and runs through Downtown Boston in the Washington Street Tunnel. The line returns to the surface in the South End, then follows the Southwest Corridor southwest in a cut through Roxbury and Jamaica Plain to Forest Hills 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.