These websites are excellent gateways to many other railway-themed websites.

Railserve.com
http://www.railserve.com/
This Internet Railroad Directory offers over 12,000 rail-related links, search engines, rail news, free classified ads, Train Simulator add-ons and more!

NMRA Directory of World Wide Rail Sites
http://www.cwr.com/
The National Model Railway Association maintains this excellent directory of over 4.900 links to railway themed sites around the world.

Railroaddata.com
http://www.railroaddata.com/
This directory maintains links to over 5,000 railroad websites.