The Market Street Subway is a double-decker subway tunnel that carries Muni Metro and BART train traffic in San Francisco, California. It runs under the length of Market Street between Embarcadero Station and Castro Street 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 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. The Embarcadero portal, which opened in 1998, is not an original part of the Market Street Subway – it is currently used by just the N Judah and the T Third Street lines.