Simutrans

GameHippo.com (9 out of 10)

The game has graphics good enough to get the job done, and simple enough to allow it to run on slower machines. The controls are mouse-driven, which makes for a very simple scheme to learn. If you are in to sim-games like Transport Tycoon or SimCity, then this will be an excellent download for you.

Curly's World Of Freeware (4 out of 6)

This I have found to be somewhat similar to Transport Tycoon, but it's free! A little hard to learn, but there is a manual on the site.