Metropolitan areas throughout the United States had popular forms of mass transit including subways and trolley lines. Following World War II many of these were replaced or supplemented by bus lines and highways. During the past few decades, cities again have begun implementing light rail systems.

Currently celebrating its centennial, the San Francisco Municipal Railway (SF Muni or Muni) is the public transit system for the city and county of San Francisco consisting of 54 bus lines, 17 trolley bus lines, 3 cable car lines, a heritage streetcar line, 7 light rail lines that operate above ground and Muni Metro the City's lone subway tube.

Following national trends, by the end of WWII Muni began replacing most of its rail lines with trolleybus service (Muni introduced its first trolleybus line in 1941). A few lines with dedicated right-of-ways (including those serving the Twin Peaks and Sunset tunnels) continued as rail lines into the 1970s. These became the foundation of the Muni Metro. Construction on BART's Market street tunnel started in 1967 but major cost overruns in the BART project forced reductions to planned service within San Francisco. As a result, the partially constructed BART stations formed the basis of a new light-rail Metro to connect the downtown stations to the Twin Peaks Tunnel and elsewhere. Construction on the metro began in 1970, but the design suffered from further cuts and design changes throughout the 1970s. The Muni Metro finally opened in July 1980. Currently, in ridership Muni is the seventh largest transit system in the United States. 1

1.     San Francisco Municipal Railway, Wikipedia, accessed 19 December 2012