The S Shuttle is a light rail service on the Muni Metro system in San Francisco, California. The service began in 2001 as the S Castro Shuttle, an effort to reduce crowding at Castro station. It was briefly discontinued in 2007 when the T Third Street line was opened. Service was extended to St. Francis Circle station in 2013, but cut back to West Portal station in 2016. In 2020, it was changed to full-time service as part of a reconfiguration of Muni Metro service. The designation of S Shuttle is also given to trains at other hours and locations, most commonly to those which run service before and after San Francisco Giants and Golden State Warriors games to provide additional capacity to Oracle Park and Chase Center respectively. History After the installation of automatic train control in 1998, the maximum Muni Metro frequency through the Market Street subway doubled from 24 trains per hour to 48. Muni needed to increase capacity to accommodate growing ridership, but the aging Boeing LRV fleet and issues with the new Breda fleet left too few vehicles to run additional service on the branch lines. Instead, Muni introduced the S Castro Shuttle, which could relieve crowding at busy Castro station while using only a small amount of rolling stock. The service began on April 2, 2001, running only at rush hour on weekdays, with headways between 7 and 12 minutes. A variety of service changes took place with the introduction of full service on the T Third Street line on April 7, 2007. The new line was routed through the subway to Castro station, replacing the S Castro Shuttle; changes were also made to the J Church, N Judah, and several bus lines. (The S Castro Shuttle designation continued to be used for extra service to AT&T Park on San Francisco Giants game days.) The changes were unpopular with the public; the Muni Metro changes caused severe delays in the Market Street subway, and forced many riders to transfer to reach the Caltrain station when they previously did not. On June 30, 2007, Muni reversed several of the changes. The J, N, and S were restored to their previous routes, while the T was interlined with the K Ingleside line. The revived shuttle ran on 10-minute headways using three trains. On October 4, 2013, Muni began a pilot of S Shuttle service, which included