The Dorchester Tunnel to Washington Street and South Station Under opened on April 4, 1915 and December 3, 1916, with transfers to the Washington Street Tunnel and Atlantic Avenue Elevated, respectively.
 Further extensions opened to Broadway on December 15, 1917 and Andrew on June 29, 1918, both prepayment stations for streetcar transfer.
 The Broadway station included an upper level with its own tunnel for streetcars, which was soon abandoned in 1919 due to most lines being truncated to Andrew.
 The upper level at Broadway was later incorporated into the mezzanine.

Next came the Dorchester extension (now the Ashmont branch), following a rail right-of-way created in 1870 by the Shawmut Branch Railroad.
 In 1872, the right-of-way was acquired by the Old Colony Railroad to connect their main line at Harrison Square with the Dorchester and Milton Branch Railroad, running from the Old Colony at Neponset, west to what is now Mattapan station.
 The New York, New Haven and Hartford Railroad succeeded the Old Colony in operating the branch, but passenger service ceased on September 4, 1926, in anticipation of the construction of the BERy's Dorchester Extension.

The BERy opened the first phase of the Dorchester Extension, to Fields Corner station, on November 5, 1927, south from Andrew, then southeast to the surface and along the west side of the Old Colony mainline in a depressed right-of-way.
 Columbia and Savin Hill stations were built on the surface at the sites of former Old Colony stations.
 The remainder of the extension opened to Ashmont and Codman Yard on September 1, 1928, and included Shawmut station, where there had been a surface Old Colony station, but where the new rapid transit station was placed underground.
 The first phase of the Ashmont–Mattapan High Speed Line opened on August 26, 1929, using the rest of the Shawmut Branch right-of-way, including Cedar Grove station, and part of the old Dorchester and Milton Branch.

Charles was renamed Charles/MGH in December 1973, and Kendall was renamed Kendall/MIT on August 7, 1978.
In January 1981, the MBTA proposed to close the Ashmont branch on Sundays – and the Mattapan Line at all times – beginning that March due to severe budget issues.
The closure was cancelled, though the lines were closed from June 20, 1981, to January 16, 1982, for track replacement and tunnel repairs.
The Dorchester Tunnel to Washington Street and South Station Under opened on April 4, 1915 and December 3, 1916, with transfers to the Washington Street Tunnel and Atlantic Avenue Elevated, respectively.  Further extensions opened to Broadway on December 15, 1917 and Andrew on June 29, 1918, both prepayment stations for streetcar transfer.  The Broadway station included an upper level with its own tunnel for streetcars, which was soon abandoned in 1919 due to most lines being truncated to Andrew.  The upper level at Broadway was later incorporated into the mezzanine.
Next came the Dorchester extension (now the Ashmont branch), following a rail right-of-way created in 1870 by the Shawmut Branch Railroad.  In 1872, the right-of-way was acquired by the Old Colony Railroad to connect their main line at Harrison Square with the Dorchester and Milton Branch Railroad, running from the Old Colony at Neponset, west to what is now Mattapan station.  The New York, New Haven and Hartford Railroad succeeded the Old Colony in operating the branch, but passenger service ceased on September 4, 1926, in anticipation of the construction of the BERy's Dorchester Extension.
The BERy opened the first phase of the Dorchester Extension, to Fields Corner station, on November 5, 1927, south from Andrew, then southeast to the surface and along the west side of the Old Colony mainline in a depressed right-of-way.  Columbia and Savin Hill stations were built on the surface at the sites of former Old Colony stations.  The remainder of the extension opened to Ashmont and Codman Yard on September 1, 1928, and included Shawmut station, where there had been a surface Old Colony station, but where the new rapid transit station was placed underground.  The first phase of the Ashmont–Mattapan High Speed Line opened on August 26, 1929, using the rest of the Shawmut Branch right-of-way, including Cedar Grove station, and part of the old Dorchester and Milton Branch.
Charles was renamed Charles/MGH in December 1973, and Kendall was renamed Kendall/MIT on August 7, 1978. In January 1981, the MBTA proposed to close the Ashmont branch on Sundays – and the Mattapan Line at all times – beginning that March due to severe budget issues. The closure was cancelled, though the lines were closed from June 20, 1981, to January 16, 1982, for track replacement and tunnel repairs.
Red Line (MBTA)