The first is under the main page for TRS04. You can find it here. http://www.auran.com/TRS2004/community.htm. I didn't count how many sites are listed there, but there have to be around 100.
The second is called RailServe. It's sub-page for Trainz is here www.railserve.com/computers/trainz. This site also has a ton of other materials relating to railroading: virtual, model, and prototype.
There's lots of sites for American stuff, it would help if you could clarify your interests more. Payware vs freeware, railroads, content looking for. I spent a couple of days clicking through links from the TRS2004 page and bookmarked a bunch.
