1973. Connections through the Transbay Tube were opened for revenue service on September 16, 1974. Embarcadero station was opened about two years later on May 27, 1976. Starting on February 18, 1980 and continuing for over a year afterward, Muni replaced surface operations with subway service at all stations in the tunnel. At that point, there were no plans to maintain the street-running tracks on Market Street, but that changed in the late 1980s after the success of several Historic Trolley Festivals. Muni's Embarcadero portal opened in 1998, and was not an original part of the subway's plan; this allowed connections to Caltrain's 4th and King Station. Between its opening in 2007 and its move to the newly-constructed Central Subway in 2023, Muni ran the T Third Street through the Market Street Subway as a merged metro line with the K Ingleside. The Central Subway connects with the Market Street subway under Union Square, as an underground walkway connects the new Union Square/Market Street station with Powell Station. Services Prior to March 30, 2020, the K Ingleside, L Taraval, M Ocean View, and T Third Street Muni Metro lines ran through the entire length of the subway to its direct connection with the Twin Peaks Tunnel. The J Church and N Judah lines left the subway via the Duboce portal at Church and Duboce streets, and only the N Judah and the T Third Street lines continued past the Embarcadero portal. Since the Muni Metro platforms at Montgomery Street, Powell Street, and Civic Center are long enough to hold two two-car trains simultaneously, the San Francisco Municipal Transportation Agency began double-berthing in April 2015. Under this process, two trains are in the station at once: the rear train discharges passengers while the front train boards passengers. The new practice was aimed at reducing passenger frustration, though it would not reduce travel times. In 1997, work began to install the SelTrac CBTC-based signalling system in the subway. The work was completed in 2001 and station-to-station operation is now completely automated under normal operation. On March 30, 2020, Muni Metro service, including trains through the Market Street Subway, was replaced with buses due to the COVID-19 pandemic. Rail service returned on August 22, with the routes reconfigured to improve reliability in the subway. No J, K, or L service entered the subway: J Church service ran only on the surface between Balboa