If your computer is any good, get Rail Simulator from Kuju. its now 7.69 (as a reboxed but identical edition) on Amazon
http://www.amazon.co.uk/Electronic-A.../dp/B001JQKT6O
You can get MSTS too for same price, but it really shows its age in comparison and isnt massively stable once you put addons on. Theres not as many addons yet for Rail simulator but its much more immersive and realistic anyway.