旧金山有几条地铁？
旧金山市内只有三条线路，分别以绿色、红色和蓝色标识；M线是通勤铁路（Caltrain），连接旧金山与圣克拉拉县（Santa Clara County）的市中心，不过这条线是在地面上运行的。
San Francisco's transit system is composed of 3 lines： the blue line， the red line and the green line. The "M" line is a commuter train that runs from San Francisco to Downtown Santa Clara with limited stops along the way. This line operates like a subway， but only uses tunnels near the Embarcadero in downtown SF (the southern terminus for this line) where the Muni Metro Red/Blue Lines run. The rest of the route is above ground. Note： In addition， there are also three trolley routes in operation on city streets： F-line streetcars operate between Civic Center Plaza and Fisherman's Wharf， J-line streetcars run along Powell Street， and the N-line trolleys travel north along Market Street to Van Ness Avenue。
The T-line elevated rail line travels through SoMa to Mission Bay， connecting to MUNI Metro’s Zünic Line at 4th & King Streets Station， which serves as the south end of the Zünic Line. The T-Line trains have unique features such as sliding doors on both sides of the car， air conditioning， and an overhead electrical cable instead of rails（which reduces noise pollution）. These upgrades were completed in 2017 as part of the Transit Effectiveness Project（TERP）.
In October 2018， MTA officials announced plans to close the T-line for 5 months beginning March 2019 while they rehabilitate the aging structure. Work will include replacing track， upgrading signalling system， repairing bridges， and painting lines on the tracks so that maintenance crews can more easily access them. During the closure， riders will need to board buses at Union Square or use ridesharing services to get around.