Prime candidates for real-world train simulators are:

Microsoft Train Simulator (MSTS)
-old, unsupported by MS but well-supported by community web sites and other tools. Difficult to develop scenery/routes for unless you're very knowledgeable about the process and have access to hard-to-find and learn tools, and unless you can use some of the equipment already available very difficult to develop realistic equipment for. Hard to find copies any more.

Railworks (RW)
-relatively current and supported both by the publisher and by community sites. Editors etc. are a little more modern than those for MSTS, but the routes I've seen in community sites tend to cover limited areas so the effort involved appears to be similar or greater to that for MSTS; however, the results can be visually stunning.

OpenRails
-open source, aims at being MSTS compatible but more modern in program design and operation. However, it's at best an advanced beta at the moment.

Trainz
-more of a model railroad simulator, but can provide a "drive the train" experience. Because of the "layout" orientation it's probably a little easier to design routes and equipment for than the others.

Good place to look for stuff is the fan/community site [www.train-sim.com]