Simutrans

Simutrans is a freeware and open-source game that simulates a transport network-style simcity
runs under Windows, Linux, BeOS on PC, and OS X on Intel based Macs.
Some features:

• You can choose many transport options: trains, airplanes, busses, trucks, ships, trams, monorails, and maglevs.
• Over 40 types of industries can appear on the map, and 35 types of goods can be transported.
• Each passenger (and unit of goods) is intelligent, knows where it needs to go, and can transfer from one vehicle to another to reach its destination.
• You can build very sophisticated stations and airports, and configure them just as you need from various structures.
• Terrain modification possibilities are almost limitless, do what you need or can afford.
• You can choose from wide variety of graphics sets.

http://www.simutrans.com