North American metro rail, subway & light rail systems - including subways, els, peoplemovers, subte networks:

Atlanta
Baltimore
Boston
Buffalo
Calgary
Charlotte
Chicago
Cincinnati
Cleveland
Dallas

Memphis
Mexico City
Miami
Milwaukee
Minneapolis/St. Paul
Monterrey
Montréal
Morgantown
New Orleans
New York City
Norfolk
Northern New Jersey (Newark)

SUBWAYS & LIGHT RAIL
in the U.S.A.
-- The Book! --
Vol. 1: East Coast

Click here for more info!

SUBWAYS & LIGHT RAIL
in the U.S.A.
-- The Book! --
Vol. 2: The West

Click here for more info!

SUBWAYS & LIGHT RAIL
in the U.S.A.
-- The Book! --
Vol. 3: Midwest & South

Click here for more info!

Cities with metro & light rail projects - planned or under construction:

Austin | Hamilton | Mississauga | Omaha | Québec | Santa Ana, Orange County

Cincinnati's Abandoned Subway

Heritage & classic streetcar systems with regular daily service:

El Paso | Little Rock | Memphis | New Orleans | Philadelphia | San Francisco | Tampa

List of rail transit systems in the United States at Wikipedia

NYC Subway Resources is the site you need for anything related to North America not included in UrbanRail.Net (incl. many good maps)

ERA (Electric Railroaders’ Association - USA)

Alamys.org - Association of Latin American Metros and Subways

Jon Bell's Transit Pages include links to many US mass transit systems

RailServe: The Internet Railroad Directory - Over 2,000 rail-related links, search engine, forums, railfan chat rooms, and more!