page_title: Trolleybuses in Greater Boston
section_title: Silver Line
hierarchical_section_title: Trolleybuses in Greater Boston / Silver Line
caption_attribution_description: English: An outbound bus at the island platform at Silver Line Way
context_page_description: The Boston-area trolleybus system forms part of the public transportation network serving Greater Boston in the U.S. state of Massachusetts. It opened on April 11, 1936, and since 1964 has been operated by the Massachusetts Bay Transportation Authority. It currently includes two physically isolated networks: one serving the towns of Cambridge, Belmont, and Watertown, the other – the Silver Line – located in the city of Boston proper. Prior to 1964, several additional trolleybus lines were in operation in Boston proper. Measured by fleet size, the system was the third-largest trolleybus system in the United States at its peak, with only the Chicago and Atlanta systems having more trolleybuses than Boston's 463.
  In the present system, four routes fan out from the Harvard bus tunnel at Harvard Square station, running through Cambridge, Belmont, and Watertown. Those lines are the remains of a once-extensive system of trackless trolleys in the area, which was largely formed from former streetcar lines. Additionally, the 2004-opened Silver Line is a bus rapid transit service using dual-mode buses which run as trolleybuses in the Waterfront tunnel.
context_section_description: The Silver Line is split into two sections; the diesel-electric hybrid Washington Street section runs entirely on the surface, but the Waterfront section runs through a dedicated tunnel under the South Boston Waterfront, in which diesel emissions would be problematic. 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), so the Waterfront section uses dual-mode buses. The vehicles run as trolleybuses within the tunnel, then change to diesel power at Silver Line Way, and reverse the transition when inbound.  Motor buses are not permitted to operate inside the South Boston Waterfront tunnel due to insufficient ventilation.
  Four routes run in the electrified Silver Line tunnel: SL1 to Logan Airport, SL2 to Design Center, SL3 to Chelsea, and a shuttle route which short turns at Silver Line Way.
  Until early 2006, the Silver Line also used some of the then-new Neoplan trackless trolleys until the entire dual-mode fleet could be placed into service. Later that year, the Neoplan trackless trolleys were transferred to the Harvard-based routes to replace the then-remaining Flyer trolleybuses.
caption: A Silver Line dual-mode bus changes from electric to diesel mode at Silver Line Way
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