The Boston-area trolleybus (or, as known locally, trackless trolley) system formed part of the public transportation network serving Greater Boston in the U.S. state of Massachusetts.

The system was expanded by the Silver Line (Waterfront), a 2004-opened bus rapid transit network using dual-mode buses which ran as trolleybuses in a tunnel in the Seaport District of Boston before switching to diesel power to serve other destinations.

[3] In 1947, the BERy was succeeded by the public Metropolitan Transit Authority (MTA) as the operator of Boston's urban transit system,[3] and in 1964, the MTA was replaced by the Massachusetts Bay Transportation Authority (MBTA), which remains the system's operator today.

What became the Harvard-based trolleybus system began as branches of the Cambridge Railroad, a horsecar street railway that opened in 1856.

[16] Electric service between Watertown and Mount Auburn began on December 12, 1893; the line was double-tracked later in the decade.

[26][27] A branch to Belmont Center opened on June 30, 1906; it was replaced by buses in August 1928 and was never a trackless trolley route.

[9] Starting in 1957, trackless trolleys provided short-turn service from Harvard to Benton Square on the route to Waverley.

[33]) In January 2005 most route 77A (Harvard–North Cambridge) service was eliminated; the only remaining trips were pull-ins and pull-outs to take 71 and 73 buses to or from the carhouse.

[10] In March 2013, route 72 trackless trolleys were replaced with diesel buses to permit roadwork on Huron Avenue and reconstruction of the Conductor's Building in Bennett Alley.

[35][36] In 2021 the MBTA announced plans to renovate the North Cambridge Carhouse facility by 2023 to accommodate battery electric buses, eliminating trolleybus operations on all Harvard-based routes.

[40][41] Multiple utility and road-rebuilding projects beginning in early 2022 led MBTA to decide to substitute diesel hybrid buses for trolleybuses at that time.

Several routes branch out from the tunnel along routings where installation of overhead trolley wire would have been impractical, especially through the Ted Williams Tunnel (as overhead wires are expressly prohibited on interstate highways[citation needed]), so the Waterfront section uses dual-mode buses.

The vehicles ran as trolleybuses within the 1.3-mile-long (2.1 km) tunnel and for about 900 feet (270 m) on the surface, then changed to diesel power at the end of Silver Line Way, and reversed the transition when inbound.

[44] Motor buses are not permitted to operate inside the South Boston Waterfront tunnel due to insufficient ventilation.

[43] Only three of the 32 dual-mode buses on order had been accepted for service by the time the Silver Line opened,[45] but the number had increased to 28 by February 2006.

[43] In fall 2005 (five vehicles)[46] and the first half of 2006, the 12 Neoplan trackless trolleys were transferred to the Harvard-based routes to replace the then-remaining Flyer trolleybuses operating out of North Cambridge Carhouse.

Regular use of Flyers ended in June 2006, but a small number remained active and used very sporadically until March 2007.

[52] The only subsequent – and the final – use occurred on December 21, 2007, when the last two active Flyers (4006 and 4016) entered service briefly after all-night sleet-cutting duty (scraping ice off of the overhead wires).