Eurostar
London to Paris in three hours by rail.
http://www.eurostar.com/

Paris.org
Find out more about the history and architecture of Paris' train stations.
http://www.paris.org/Gares/

PIDF.com
Useful tips and information about train travel in France.
http://www.pidf.com/an-world/u_1/home12.asp

PIDF.com
Basic information about Paris' major railway stations.
http://www.pidf.com/an-world/u_1/home123.asp

RATP
An official guide to the RER - the suburban rail system in Paris.
http://www.ratp.fr/index_eng.htm

SNCF
The website of the France's main rail operator contains train timetables for Paris and the rest of the country, as well as plenty of other information.
http://www.sncf.com/

Thalys
A train service that connects Paris, Brussels, Amsterdam and Cologne.
http://www.thalys.com/be/fr/welcome