By 1896, electric routes ran from Newton Corner (Nonantum Square) to downtown Boston via Brighton and via Watertown.

The line was operated until 1969, when it was replaced by MBTA bus route 57, though it remained in service for non-revenue moves to Watertown Yard until 1994.

[15] A lengthy Nonantum Square–City Point route via Allston, Coolidge Corner, and Park Square opened on August 4, 1900.

[18] The Boylston Street subway was opened to an incline in Kenmore Square on October 3, 1914, with Watertown service via Brighton Center immediately routed into the new tunnel.

[21] The Blandford Street portal, on Commonwealth Avenue west of Kenmore, opened October 23, 1932, marking the last change in the route of the service.

[18] In June 1894, the West End applied for city permission to construct a line on North Beacon Street between Union Square and the Watertown border.

After approval by the Watertown Arsenal (which owned a section of North Beacon Street) in 1896 – despite the objection of the West End – the company constructed the line in 1898.

[23][24][25] After further disagreements between its parent company Newton and Boston Street Railway and the BERy about operations, the line opened in 1900.

[28]: 50  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.

[40] By the late 1960s, wear and tear and a lack of replacements caused a shortage of Green Line streetcars, and the MBTA announced the closure of the A branch on a "trial" basis.

The legacy tracks remained after 1969, being used for several decades to allow Green Line streetcars to reach the Watertown Carhouse for maintenance.

[39] Additional overhead infrastructure, as well as the tracks themselves, were mainly removed by the end of the summer of 1996 as part of a resurfacing project on North Beacon Street.

The stub, part of the former outbound track, was used for temporarily storing disabled trains until it was shorted to just a few feet long in the mid 2000s; the switch was not disconnected until 2014 and not paved over until 2020.

Restoration of the "A" branch was advocated to provide a one-seat ride to downtown Boston from the northern corridor (rather than requiring a transfer at Kenmore), but was ruled too costly and less safe compared to the replacement bus service.

[44] Additional short turn trips between Oak Square and Kenmore at rush hour were cut in June 1971, and restored for mornings only in 1987.

[39] On September 1, 2008, due to overcrowding on the inner part of the route, the MBTA began operating the 57A Oak Square–Kenmore station short turn of the 57 during weekday rush hours.

Oak Square short turn service was discontinued in December 2020 except for a small number of school-day trips; frequency on the full route was increased.

[39] A 2018–19 MBTA review of its bus system found that route 57 suffered from overcrowding at peak hours and poor reliability.

[48] 2022 MBTA proposals for its systemwide bus network redesign called for the 57 to have increased frequency to provide service every 15 minutes or better at all operating hours.

[39] In September 1968, the 61 was changed to originate at Brighton Center, from where it ran through Oak Square and entered the Turnpike at Newton Corner.

(The 300-series numbers had been used by ex-Eastern Massachusetts Street Railway bus routes in Lawrence, briefly under MBTA control, in 1968).

[53] The MBTA recommended that outbound route 502 and 504 buses operate directly to Galen Street, rather than looping the Newton Corner rotary to serve those stops.

[48] This was implemented on September 1, 2019; outbound route 501 and 503 buses began serving the Newton Corner stops at that time.

[56][57] The May 2022 network proposals called for the 2020 changes to be made permanent, and for routes 501 and 504 to only operate at weekday peak hours.