Rail3d is a railway network simulation being developed by Mark Goodspeed. The focus is on reproducing operational aspects, especially signalling, as accurately as possible (Mark G is a volunteer signalman on a steam railway), and for a time at least Rail3d was much better at that sort of thing than most of its commercially developed rivals. Of course, it couldn't match them on performance and quality of the graphics, and with their big user communities they eventually got nearly as good as Rail3d at signalling too.
Rail3d swallowed a lot of my free time over the years - and lets be honest, I enjoyed it - but I've only been very marginally involved since I started building a "real" model railway. It is very impressive what we could do as a bunch of amateurs, but of course commercial rail simulators are getting better all the time too.
This site used to serve as repository for a lot of Rail3d information, but that has now moved to the official site rail3d.net, where you can also find the latest versions of the software and models, including some of mine.