Several companies originally developed cable car lines and electric street railway lines, which by 1902 had consolidated into United Railroads of San Francisco. In 1921 that company became the Market Street Railway, which after 1925 was a subsidiary of Standard Gas & Electric Co. In 1912, the City began operating its own streetcar lines under the name San Francisco Municipal Railway (Muni). In 1944, Muni acquired the Market Street Railway. And in 1952, complete unification was finally achieved, when Muni acquired the independent California Street Cable Railroad Co. Muni continues to operate cable cars, streetcars, trolleybuses, and motor buses.
The San Francisco Cable Car Website
Jon Bell's Rail Transit Pages/San Francisco
Market Street Railway Online
San Francisco Municipal Railway (Wikipedia)
San Francisco Municipal Railway Fleet (Wikipedia)