Electric trolley buses 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 can be less flexible than that of motor buses, trolley buses 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 buses have a battery which allows them to travel off-wire and reroute around anything blocking their path, such as an excavation site or a street fair. The use of trolley buses 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 Muni has the largest trolley bus fleet of any transit agency in the United States and Canada. Muni’s trolley coaches (as well as its streetcars and cable cars) are almost entirely pollution-free, since their electric power comes from the city's hydroelectric Hetch Hetchy Water and Power System.
Muni currently has two generations of trolley bus fleets in service as one phases out the other.
The two fleets were manufactured by: