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 (shown as part of the Red Line on maps, but operated separately) continues to Mattapan station. The Braintree branch runs southwest through Quincy and Braintree to Braintree station. The Red Line operates during normal MBTA service hours (all times except late nights) with six-car trains. The 218-car active fleet consists of three orders of cars built in 1969–70, 1987–89, and 1993–94. A 252-car order from CRRC is being built from 2019 to 2024. The Red Line is fully grade-separated; trains are driven by operators with automatic train control for safety. Cabot Yard in South Boston is used for heavy maintenance and storage; yards at Alewife, Ashmont, and Braintree are also used for storage. All 22 Red Line stations are fully accessible. Averaging 240,000 weekday passengers in 2019, the Red Line has the highest ridership of the MBTA subway lines. The Boston Elevated Railway opened its Cambridge tunnel between and in 1912. It was extended south as the Dorchester Tunnel to Washington (now ) in 1915, in 1916, in 1917, and in 1918. The Dorchester extension added three stops to in 1927 and two more stops to Ashmont in 1928. Charles (now ) was added as an infill station in 1932. The newly formed MBTA assigned colors to its subway lines in 1965, with the Cambridge–Dorchester line becoming the Red Line. The MBTA added the three-station South Shore Line to in 1971; it was extended to Braintree in 1980, with added as an infill in 1983. The Red Line Northwest Extension, originally planned to run to Arlington Heights or Route 128, opened to in 1984 and Alewife in 1985. History Cambridge tunnel The Red Line was the last of the four original Boston subway lines (the others being the Green, Orange, and Blue Lines, opened in 1897, 1901, and 1904, respectively) to come into being. Construction of the Cambridge tunnel, connecting Harvard Square