Trolley coaches (also known as “trolley buses” or “trackless trolleys”) are rubber-tired vehicles with motors powered by electricity from overhead wires. “Trolley” refers to the trolley poles on the roof of the bus that are used to transmit the electricity from the overhead wires.
Although their operations are less flexible than that of motor buses, trolley coaches are more energy efficient, much quieter and much less polluting. They operate better on hills, require less maintenance and are longer lasting than motor buses. Modern trolley coaches have an auxiliary power unit (APU), which allows the buses to travel off-wire for several blocks and avoid anything blocking their normal route, such as an excavation in the street or a street fair. The use of trolley coaches is generally restricted to lines on which a high-enough frequency of service can justify the expense of the electric power system installation and vehicle costs.
San Francisco has the largest trolley coach fleet of any transit agency in the U.S. and Canada. San Francisco's trolley coaches (as well as its streetcars and the cable motors for the cable cars) are almost entirely pollution-free, since their electric power comes from the city's hydroelectric Hetch Hetchy Water & Power Project. For many people, trolley coaches' quieter, cleaner service outweighs the unsightliness of the overhead wires necessary for their operation.