transit line along the corridor. The line was expected to be completed within two years. The agreement also provided for the MBTA to subsidize commuter service on the railroad's remaining commuter rail lines for $1.2 million annually. Original plans called for the South Shore line to be largely independent of the existing Red Line, with either a northern terminus at the surface level at South Station or a tunnel leading to a stub-end terminal between Post Office Square and State Street. However, it was later decided to have the line be a new southern branch of the Red Line. The first section of the South Shore line, under construction since 1966, opened on September 1, 1971, branching from the original Red Line at a flying junction north of Columbia (now JFK/UMass). It ran along the west side of the Old Colony rail right-of-way (which has since been reduced to one track), crossing to the east side north of Savin Hill. The northernmost station was North Quincy, with others at Wollaston and Quincy Center. Service began alternating between Ashmont and Quincy. Ashmont service operated with 1400-series cars, while the Quincy branch only had 1500- and 1600-series cars because they had cab signaling. In December 1969, the MBTA purchased Penn Central's Dover Street Yards for $7 million. The site was used for the South Bay Maintenance Center (later Cabot Yard), which included Red Line shops (to replace Eliot Yard) and an adjacent bus garage. A $7.8 million construction contract was awarded in 1972, with groundbreaking on September 16. The facility was dedicated on June 24, 1974; on December 28, Bartlett Street garage in Roxbury was closed. Three southbound trains collided inside the Beacon Hill tunnel on August 1, 1975, injuring 132 passengers. Braintree extension Beyond Quincy Center, the Braintree extension runs southward to Braintree, opened on March 22, 1980, via an intermediate stop at Quincy Adams which opened on September 10, 1983 due to delays. The extension was part of the massive 1965 extension plan, although it was delayed due to questions over station siting in Braintree. The Boston Transportation Planning Review, published in 1969, proposed North Braintree and South Braintree stations following the Quincy Center station. Several outlying sections of the MBTA subway system, including Quincy Adams and Braintree, originally charged a double fare to account for the additional costs of running service far from downtown. Passengers paid two fares to