Train Sim is really for if you just wan't to drive pre-made scenario's and enjoy the routes, but Trainz is hardcore if you want to design, build and run a railway. The amount of freeware that is available is staggering, and you get access to it all. there is also a Steam version available, although I always get mine as a digital download from Auran - I do not wan't to be reliant on Steam for it.
At a guess, I would say that I have spent more time on Trainz than all my other PC games in total.
“O Lord God, let me not be disgraced in my old days.”
'We few, we happy few, we band of brothers'