An electric trolleybus (also known as trolley bus or trackless trolley or simply trolley) is a bus powered by two overhead electric wires, from which the bus draws electricity using two trolley poles (one for the return current, which can not pass to the ground as in the case of a tram). Unlike light rail, trolleybuses use rubber tires rather than steel wheels on rail.
Some cities, led by the Brooklyn-Manhattan Transit Corporation (BMT—New York), subscribed to the all-four concept of using buses, trolleybuses, trams (in U.S. called streetcars, trolleys or light rail) and rapid transit subway and/or elevated lines (metros) as appropriate for routes ranging from lightly-used to heaviest trunk line. Buses and trolleybuses in particular were seen as entry systems that could later be upgraded to rail as appropriate. Although the Brooklyn system under the BMT only built one trolleybus line, other cities, notably San Francisco, California and Philadelphia, Pennsylvania, built larger systems and still maintain "all-four." If you include cable cars as another mode, San Francisco could be called "all-five," as the cable cars do provide a general transportation function in addition to being a tourist attraction.
Like other electric vehicles, trolleybuses are often seen as more environmentally friendly than hydrocarbon based vehicles such as buses, but the power is not "free", and instead has to be produced at centralised power plants. On the other hand, centrally-produced power has the advantage of being more efficient, not bound to a specific fuel source, and more amenable to pollution-control as a single-source supply than individual vehicles, each with its own power generation.
One advantage that is rarely used in other vehicles is that they can generate electric power from kinetic power whilst braking, a process known as regenerative braking.
Another area where trolleybuses enjoy a special niche is locations where hydropower is abundant and cheap. Examples of this are the extensive trolleybus systems in Vancouver, British Columbia, Canada and Seattle, Washington, USA. The latter city doubly benefits, due to the steep road grades near the Downtown waterfront as well as on Queen Anne, First, and Capitol Hillss.
While at one time numerous cities operated this mode of transport, it is uncommon today in North America and not expanding in Europe, though it is still a common form of transport in the former Soviet republics, generally occupying the niche between street railways and petroleum-powered buses.
