Simutrans is a free and open-source transport simulation game. The program allows you to construct and operate many different transport types: trains, planes, ships, monorails, maglevs and more. Not all at once, of course - you'll start in 1880, finish in 2050 - and not just randomly, either.
Your aim is to provide the transport links your people need, so for instance to make sure that the refinery gets regular supplies of oil, the power plant receives coal, the furniture factory has wood, and the bookstore receives new books as required. When all goes well everyone's happy and your city grows, but if you're not paying attention... Well, it's a very different story.
There are lots of options to master here: more than 40 types of industries may appear in your cities, for instance; more than 35 types of goods may need to be transported; and you don't just build identical stations or airports, each of them may be configured in various ways to optimise your use.
The program can be further extended with multiple "PakSets", each of which have their own graphics for landscapes, buildings, vehicles and more. You'll see some of these listed in the installation program and there are online images here.
And once you've spent some time figuring out how everything works (which really is going to take a while - this isn't for casual gamers) then you can play against various computer AI players or even online.
Simutrans has plenty of depth, so initially you'll just be spending loads of time learning exactly what you can do, and how to do it. If you enjoy this type of simulation then it's worth the effort, though - there's a lot of fun to be had here.