The Market Street subway is a two-level subway tunnel that carries Muni Metro and BART trains under Market Street in San Francisco, California. It runs under the length of Market Street between Embarcadero station and Castro station. The upper level is used by Muni Metro lines and the lower level is used by BART lines. BART does not run through the whole subway; it turns south and runs under Mission Street southwest of Civic Center/UN Plaza station. The northeastern end of the BART level is connected to the Transbay Tube. On the Muni Metro level, the southwestern end of the Market Street subway connects to the much-older Twin Peaks Tunnel, and the northeastern end connects to surface tracks along the Embarcadero. History The Market Street Railway had existed on the surface parallel to the subway's alignment in some form since 1860 with services terminating at the Ferry Building. To alleviate traffic, plans for a tunnel under Market Street can be dated to at least 1912. By 1918, there were four tracks running down the thoroughfare — two per direction. That plan is nearly identical to the design built 60 years later, including two levels of train traffic and provisions for both overhead and third-rail power delivery, but not accounting for a Transbay Tunnel. The Twin Peaks Tunnel was built with the east end sloping downward, foreseeing future connection to a tunnel under Market Street. Serious consideration for construction was finally given while designing the initial BART system. To earn the votes of San Francisco residents, the San Francisco Bay Area Rapid Transit District planned to build a two level tunnel with BART trains on the lower level and Muni streetcars on the upper level. However, the entire tunnel would be owned by the district and it designed Muni’s level with features like very long platforms to allow it to be converted to use by BART trains. However, the district would not purchase new streetcar vehicles compatable with the tunnel and did no planning on how to integrate the existing lines into the tunnel before making their pitch to voters. Construction, commencing in July 1967, was carried out via the cut-and-cover method. Embarcadero station was added later in the planning and construction process, so only the basic structure of the station was built initially. BART trains first ran through the subway with service as far as Daly City on November 5,