OpenTTD is an open-source business simulator that's heavily based on the 1995 Transport Tycoon Deluxe. As you might imagine from that brief description alone, OpenTTD is all about trying to become ridiculously wealthy by transporting passengers and cargo through a network of railways, roads and rivers, ideally without getting your vehicles stuck sideways on either of them!
While there is a bit of a learning curve, OpenTTD is a highly enjoyable and surprisingly relaxing game once you get into it. So if you're curious about the whole thing, you might be happy to hear that it has just recently made its way to Steam, and much like everywhere else, it's entirely free to check out!
Should you decide to dive in, it's also worth mentioning that one of the best things about OpenTTD is the sheer amount of mods the community has created for it over the years. These range from gameplay modifications to elaborate reskins, so once you've gotten comfortable with the base game I'd highly recommend diving deep into the modding world and customizing the entire game to suit your playstyle.
You can check out some of the more basic mods, as well as learn more about OpenTTD as a whole, over at the OpenTTD wiki. Have fun, and if you can, try to play it in multiplayer as it's just delightfully chaotic!