It has not been mentionned there apparently yet :
http://openrails.orgOpen Rails is an open source train "driving" simulator based on the old Microsoft Train Simulator from 2001, and can run all the routes and engines of the commercial game if you have it (and probably its addons too)
As mostly everything downloadable is actually commercial addons (in a similar way to the Flight Simulators one) and Open Rails by default feature no track and if you do not have MSTS to benefit from its routes, there are fortunately 2 free routes you can download to test the simulator to see if it triggers your interest enough to get into that.
http://openrails.org/download/content/I only toyed with the demo for Edinburgh towards Glasgow, it's quite nice once you manage to figure out how to start your engine
as the landscape around does not repeat itself that much so you can look around while waiting to reach each stations without being too much bored. But i'm not sure if there are some realism difficulty setting i forgot to enable, but i didn't had to really watch my speed, nearly spent all the track near max speed without any kind of problems, accident, breakage, so basically all i had to do was just watch the landscape without having to pay any attention to what was going on :/
Didn't even bothered to slow or stop at stations too, without penalties.
Unfortunately that demo has a lot of flickering z/fighting on the rails in front of your cabin by default, some people have altered the profiles height , to avoid the z fighting and remove the flickering, you can download the raised track models there :
http://digital-rails.com/track.htmland replace the default files in the demo by the fixed models (UKFineScale ones).
video of how the sim play
https://www.youtube.com/watch?v=nebeAuYXnsInote : the demo route is very likely unfinished, near the end of the route i had a lot of the landscape that was missing