The San Francisco cable cars are the only moving National Historic Landmarks. The cable car system forms part of the intermodal urban transport network operated by the San Francisco Municipal Railway, or Muni as it is better known. Cable cars operate on two routes from downtown near Union Square to Fisherman's Wharf, and a third route along California Street. While the cable cars are used to a certain extent by commuters, their low speed, small service area, and premium fares for single rides make them primarily a tourist attraction.
San Francisco's cable cars are unique in that they are the only street railway in which the cars do not operate under their own power. Instead, the cars are propelled mechanically, by "gripping" a continuously moving steel cable which runs in a conduit underneath a slot between the rails. The cable, in turn, is kept in motion by an engine in a centrally-located powerhouse.