1907. The portion west of Market Street was among the only BERy lines never to be double-tracked. The Newton Street Railway (later the Middlesex and Boston Street Railway) began Waltham–Park Street service via the North Beacon Street line on February 23, 1903. This service was rerouted to Central Square on April 27, 1912, running on Arsenal Street and Western Avenue rather than North Beacon Street. Oak Square–Boston service via Western Avenue was also cut to Central Square. Watertown–Park Street service via North Beacon Street remained. Upon the opening of the Boylston Street subway in October 1914, North Beacon Street service was cut to a Watertown–Union Square shuttle. It was further cut to a North Beacon Street bridge (the Brighton/Watertown line) – Union Square shuttle on June 2, 1917. The North Beacon Street shuttle was replaced with a bus route on February 23, 1922. A Watertown–Arsenal Street bridge line, which ran on North Beacon between Watertown and School Street, was briefly run in 1925–26. The Central Square–Union Square line was rerouted via Western Avenue and Soldiers Field Road on December 14, 1925, with a new bus route on River Street. The bus route was extended to Union Square on September 11, 1926, replacing the streetcar line. The two bus routes were combined in February 1933 to eliminate the transfer; the route is now the route 64 bus. Closure Prior to 1967, when the current letters were assigned to the Green Line's branches, the Watertown Line was route 69. (Route 57, the line's current number, was originally assigned to short-turn trips on the Huntington Avenue line.) The line was assigned the letter "A" because it was the northernmost branch. Turnback loops existed at Union Square (via Cambridge Street and Harvard Avenue) and Oak Square, as well as the terminal. Additionally, from 1915 to January 15, 1962, a loop existed at Braves Field, for special events along the part of the line concurrent with the Commonwealth Avenue line. The opening of the Riverside Line in 1959 caused streetcar lines in Cambridge running out of Harvard Square to be converted to trackless trolley, and the newly freed PCC streetcars were reassigned to the Riverside branch. However, the 1940s-era PCC fleet was aging, and maintaining five different types of PCCs complicated mechanical work. By the late 1960s, wear and tear and a lack of replacements caused a shortage of Green Line streetcars, and the MBTA