From SFWiki

Muni Metro is a light-rail system in San Francisco. All Muni Metro lines serve Market Street via a subway tunnel and radiate out to serve San Francisco's western, southern neighborhoods and connect with other Muni bus and streetcar lines.

The 6 metro lines are:

• J-Church
• K-Ingleside
• L-Taraval
• M-Ocean View
• N-Judah
• T-Third Street (limited weekend service opens January 13, 2007, full service opens April 7, 2007)

Sometimes, during peak service hours, Muni also runs a Castro Shuttle between Embarcadero Station and Castro Station. When operating the Castro Shuttle is displayed on electronic signs as the 'S' line. On April 7, 2007 the Castro Shuttle will be discontinued and replaced with the T-Third Street line.