 By World War II, most of San Francisco's streetcar routes had been converted to buses, but some lines still utilized exclusive right of ways.The upper level of the newly-constructed Market Street Subway, originally intended to carry several lines of the Bay Area Rapid Transit system, was given to the San Francisco Municipal Railway to establish the Muni Metro.Simultaneously, PCC cars were replaced with modern light rail vehicles, service rerouted from the surface, and several stations were upgraded to include high-level platforms.