Transport and logistics tycoon classic OpenTTD has a lot to offer if you love trains, boats, airports, highways, and other examples of sprawling infrastructure. Free and open source, OpenTTD has a vast log of maps, mods, graphics packs, and more for you to use. The recent release of the game on Steam has some people getting back into it, or learning its wonders for the first time.
Me? I'm really into this giant map of Europe. It's 8192x8192. That's 8192^2. That's... that's very large. It's all of Europe with a bit of North Africa, most of Turkey, and a lot of Russia. It was made by Korean OpenTTD player tiram2sue and friends and I love it. It's so big you can't see the whole thing in one screenshot at maximum zoom. The map is optimized to work with JGRPP, a release of OpenTTD. You can find it via tt-forums.net.
Anyway, I'm off to spin up an OpenTTD multiplayer server. It's gonna rule. Should only take a few thousand hours to fill this beast of a map. That is, of course, after I spent ten hours deciding on train sets and graphics packs. You can find OpenTTD via its official dedicated website and on Steam.