Interesting Links to check out:

San Francisco Municipal Railway
http://www.sfmuni.com/
Official web site of the San Francisco Municipal Railway

Market Street Railway
http://www.streetcar.org/
Official web site of the Market Street Railway (historic streetcar support group in San Francisco)

Cable Car Home Page
http://www.cable-car-guy.com/
Unofficial web site about San Francisco's cable cars, and cable car installations all over the world

SFMuniHistory
http://finance.groups.yahoo.com/group/SFMuniHistory/
San Francisco transit discussion group (hosted by Peter Ehrlich)

NYCSubway.org
http://www.nycsubway.org/
Site with images of the New York City Subway, and rail systems all over the world