Fantastic game! Back when games were made to last more than a day, nearly 14 years old and still great!
The basis is simple, build a transport company, using trains, road vehicles, aircraft and boats. And make money! It always started out simple, passengers from A to B, subsidies, money, happy days. Then you start hauling livestock, grain, coal, oil, Iron Ore, steel etc to factorys, steel mills power stations etc. Before you know it, you have this insane network all connected in a smoothly running machine, praying a train doesn't break down and kill your operation dead, lol.
A new version was released a few years back, although unofficial, the Open TTDX version is very good. With loads of improvements and a unlimited number of vehicles allowed.
I would recomend this game to anyone, and it only loses half a point because of the sleep deprevasion I suffered!