 This form of design and construction will avoid the need for subsequent rebuilding of stops on tram routes, even though both cities already have underground lines with high-floor platforms.In San Francisco, the Muni Metro light-rail system, which has both on-street and underground stations, uses a combination of high and low platforms, and the vehicles feature retractable stairs to accommodate both platform types.For on-street stations, stairs are deployed within the vehicle to allow boarding from low platforms; as the trains move underground, the stairs rise until they are flush with the floor to allow boarding from high platforms.