S Shuttle

The S Shuttle is a light rail line in the Muni Metro system in San Francisco, California. It runs through the entire Market Street subway and Twin Peaks Tunnel.
History.
Muni introduced the S Castro Shuttle. This aimed to reduce crowding at the busy Castro station. The service began on April 2, 2001. At this time it was running only at peak hours on weekdays. It was discontinued in 2007 when the T Third Street line was opened. Service went further to St. Francis Circle station in 2013, but cut back to West Portal station in 2016.
Service changes.
A lot of service changes took place with the start of full service on the T Third Street line on April 7, 2007. The new line was routed through the subway to Castro station. It replaced the S Castro Shuttle; changes were also made to the J Church, N Judah, and several bus lines. The changes were unpopular with the public; the Muni Metro changes caused big delays in the Market Street subway, and forced many users to transfer to reach the Caltrain station when they did not back then. 
Reversing changes.
On June 30, 2007, Muni reversed some of the changes. The J, N, and S were restored to their previous routes. And the T was combined with the K Ingleside line.