Bumping up an old thread...
Derail Valley has improved a lot with the Overhauled Patch -- and I absolutely love it!
The game isn't the most realistic train simulator on the market, nor does it aim to be if I'm not mistaken, but to me it definitely is the most entertaining train sim currently available. The world and game play is well thought out and beautifully created, the trains are fun, feel nicely solid and heavy, and can even be quite challenging to operate.
For me personally, the most rewarding aspect of the game are the switching duties, which get procedurally generated while you're playing the game (new jobs get created from the cars you've dropped off in an earlier task).
Another unique feature (for a train sim) is that the game doesn't have a main menu -- everything you do, be it choosing which locomotive to run today, servicing or repairing it, picking a job to run, or clearing up the mess and paying the fees for repairs after a derailment -- it all happens directly in 1st person perspective in the game world. There are no scenarios or missions, instead it's all one big continuous adventure.
More info here:
http://www.derailvalley.com/Some mods are available on
https://www.nexusmods.com/derailvalley (though not all of them have been updated for the latest patch yet)
(And no, I'm not affiliated with the devs. I just really like this game.)