OpenTTD

OpenTTD is an open-source transport-management game based on Transport Tycoon Deluxe, giving players the job of building and managing a transportation empire. You can create road networks, railways, shipping routes and air connections while moving passengers and cargo between towns and industries. The basic idea is simple, but the possibilities quickly become wonderfully complicated as your network expands and traffic begins behaving like a personal vendetta. Efficient route planning, infrastructure investment and financial management are all important, while the enormous maps provide plenty of room for ambitious projects. The game also supports multiplayer, making it possible to spend an evening discovering that your friend has somehow built a railway directly through the middle of your carefully planned empire. Civilization, but with trains.