There are two, really, worth considering for beginners.
Microsoft Train Simulator, it's quite old now and graphically not the best, but it runs on older machines well and it has lots of downloadable locomotives, stock, routes etc. Might be hard to find a copy these days.
RailWorks, the latest Rail Simulator, has nice graphics, good for beginners, but a bit more expensive. You need a pretty new computer too. It's available off the shelf or it's downloadable through Steam, a gaming platform from a game developer named Valve, for PC.
http://www.railsimulator.com/