What OpenTTD is
OpenTTD is a business simulation game where you make money by moving passengers and cargo over road, rail, water, and air. It's an open-source remake and expansion of the 1995 Chris Sawyer game Transport Tycoon Deluxe, and it's free and open-source software under the GNU General Public License version 2.0, with ongoing development. You play on procedurally generated maps of up to 4096 x 4096 tiles in one of four climates, laying down roads, railways, docks, and airports to connect industries and cities.
Playing together
You can work alone or with friends on the same map. A group builds an interconnected, multimodal network: routes hand cargo between vehicle types as needed, so one person can run the rail trunk lines while another handles the docks, airports, or last-mile road vehicles. Production chains span the map, and finished goods need to reach towns for the whole network to pay off. Steam lists Online Co-op and Online PvP among the features, so you can also play against each other rather than cooperating.
Before you start
Expect a long game with a lot of moving parts. Towns grow, new transport modes appear over the years, and industries change production or fail, so stations and network capacity have to keep expanding to avoid delays. Cities have dense streets, and maps include oceans, rivers, and high mountains to route around or through. The isometric 2D pixel look is straightforward, but planning a network that stays profitable is the real work.
The game also has a community mod scene. You download mods through an in-game content service: real-world vehicles, new industry sets, custom scenarios, and more, which changes how the game plays between sessions. OpenTTD is tagged as moddable, so what you load shapes the experience for everyone in the group.