The free, open source train simulator

Windows XP & Later
Not compatible with Windows RT!
Most Major Linux Distributions (Ubuntu, Fedora, Slackware, Etc.)
Not compatible with ARM-based systems
OS X 10.5.8 & Later, Intel Macs
Not compatible with PowerPC Macs
Developer Tools
Object Viewer, Route Viewer, Train Editor & Object Bender
Source Code 
The inner workings of openBVE is freely available
Routes, Trains, Plug-ins, etc.