Recent Power Developments
With the introduction of hybrid designs, trolleybuses are no longer tied to overhead wires. Since the 1980s, trolleybus systems in Muni in San Francisco, TransLink in Vancouver, and in Beijing, among others, have bought trolleybuses equipped with batteries to allow them to operate fairly long distances away from the wires. Supercapacitors can be also used to move buses short distances.
Trolleybuses can optionally be equipped either with limited off-wire capability—a small diesel engine or battery pack—for auxiliary or emergency use only, or full dual-mode capability. A simple auxiliary power unit can allow a trolleybus to get around a route blockage or can reduce the amount (or complexity) of overhead wiring needed at operating garages (depots). This capability has become increasingly common in newer trolleybuses, particularly in North America and Western Europe, where the vast majority of new trolleybuses delivered since the 1990s are fitted with at least limited off-wire capability. These have gradually replaced older trolleybuses which lacked such capability. In Philadelphia, new trolleybuses (known there as "trackless trolleys") that were placed in service by SEPTA in 2008 are equipped with small hybrid diesel-electric power units for operating short distances off-wire, instead of using a conventional diesel drive train or battery-only system for their off-wire movement.
King County Metro in Seattle, Washington and MBTA in Boston use or have used dual-mode buses that run on electric power from overhead wires on a fixed right-of-way and on diesel power on city streets. Metro used special-order articulated Breda buses with the center axle driven electrically and the rear (third) axle driven by a conventional power pack, with electricity used for clean operation in the downtown transit tunnel. They were introduced in 1990 and retired in 2005, replaced by cleaner hybrid buses, although 59 of 236 had their diesel propulsion equipment removed and continue (as of 2010) in trolley bus service on non-tunnel routes. MBTA uses dual-mode buses on its new (2004-opened) Silver Line (Waterfront).
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