There are several kinds of simulators. Prototype, cab operations, model layout simulation and regional dispatch simulators. Your selection will depend on the scale of operations you are trying to simulate.
BVE – Boso View Express – “Freeware Japanese train simulator; the latest version features new realism and high resolution graphics (1024×768), supports most 2.x version routes in CSV format.” Cab view simulator. OS: WinXP
LocoMotion – “Use railroads, trucking lines, buses, airplanes and ships to outmaneuver ruthless competitors and see who is best at meeting the needs of a growing metropolis.” Regional dispatch simulator. $10 + shipping. OS: WinXP
Rail3D – Model railway simulation software for building routes and running multiple trains. Model layout simulation. Freeware download. OS: WinXP
Train Dispatcher – “control the movement of trains over large track territories. This game simulates the gargantuan routing tasks a real-life dispatcher faces daily. Regional dispatch simulation. USD: $45, OS: WinXP.
Train Player – “offers bold new adventures in mouse-driven armchair model railroading. Designed for the railroader who studies track plans and dreams of an empty basement.” Model layout simulation. USD: $30-100, OS: WinXP and Mac OS X.
Microsoft Train Simulator – “brings the power and excitement of some of the world’s most famous trains to your PC” USD: $45 – possibly not a current release. Cab view simulator. OS: WinXP
Auran Trainz Simulator 2009 – a 3D train simulator boasting “more realism, more rail lines, more features, more flexibility, more community” USD: $30 Cab view simulator. OS: WinXP
Trainz for Mac now available for download from Auran site.