No trip to San Francisco is complete without a ride on the Cable Cars. Once an integral part of the municipal transportation for the City, the numerous cable car lines (eight at one time with over 600 cars) started disappearing in the forties with the proliferation of the automobile and buses. Preserving three of the lines along Powell-Mason, Powell-Hyde and California streets, the cable car system has remained essentially unchanged since its invention in 1873 by an engineer Andrew Hallidie who was inspired by watching a horrible accident when a horse-drawn tram slipped down a steep hill dragging the horses with it. These moving Historic Monuments are stored in the Cable Car Barn at night where they are meticulously maintained. The Cable Car barn also houses the powerhouse for the entire cable system that moves the authentically-replicated cars along the 17 miles (25 km) of track at a steady speed of 9.5 mph (15.5 km/h). The Cable Car Barn and Museum is open to the public where one can see the powerhouse in operation and see on display the original cable car that was used in August of 1873.