73 buses to or from the carhouse. 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. It was planned that route 72 would return to electric operation when construction was complete, but service was reduced to peak-only in December 2019 (in conjunction with improvements to routes 74 and 75 which also run on Concord Avenue), and all route 72 service was suspended indefinitely in December 2020. 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. This included shifting the North Cambridge routes to another, unelectrified garage in 2022 to permit rebuilding of North Cambridge to begin unimpeded by active operations. Accordingly, trackless trolley service on routes 71 and 73 was replaced with diesel buses (and route 77A eliminated entirely) on March 13, 2022. Vehicles In the lower/westbound tunnel, the loading platforms are located on the left side of the roadway, and for this reason the trolleybuses serving this station were equipped with left-side doors, in addition to the normal righthand doors, to avoid the need to board passengers from doors facing the wall rather than the platform. Diesel buses serving the tunnel are not equipped with left-side doors, and passengers boarding or deboarding at the westbound stop must cross the roadway to reach the buses' doors. In 2004, the MBTA began receiving a fleet of new trolleybuses from Neoplan USA, to replace Flyer E800 trolleybuses dating from 1976. The newer vehicles were low-floor vehicles and met Americans with Disabilities Act standards (see MBTA accessibility). Operations The vehicles for Harvard-based routes were all stored overnight and maintained at the North Cambridge Carhouse, the northern terminus of the 77A route. Trackless trolleys dId not run on Sundays; instead, diesel buses covered the routes. This was the case since 1963. To expedite passenger boarding through a left-side door in the southbound Harvard bus tunnel, the 71 and 73 trackless trolley routes allowed free entry for outbound trips, collecting fare upon later passenger exit from the vehicle. Silver Line The Silver Line is split into two sections; the diesel-powered 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.