I got into OpenTTD when I realized that it was a major source of inspiration for Factorio.

Some notes:

* OpenTTD is more "purely" about transportation. While Factorio has very good train-logic (block-signals vs chain-signals), OpenTTD has bridges, tunnels, terrain modifications, basic signals, path-based signals, entry/exit signals and more. OpenTTD Trains slow down going up hills and around turns: meaning optimal train pathing requires significant thought into terrain flatness and turn radius.

* OpenTTD also has planes, boats, trucks, and busses. Furthermore: log trains can only carry logs, passenger trains can only carry passengers. While the item-tree of OpenTTD is simpler, the overall logistics of transportation are far more complex. Of course, Trains are the most interesting part. (Boats have no collision detection. Trucks have very low throughput and are mainly just an early-game / simplicity option. Airplanes are relatively simple: fill them up until your airports are full)

* OpenTTD train scheduling is far deeper than Factorio. Your schedules can include target dates (ex: 30 days between TownA and TownB, with 4 days target of loading/unloading time), as well as multiple trains that share a schedule. Trains also require maintenance and begin to break down if you neglect maintenance. Learning to build high-throughput maintenance networks is part of OpenTTD.

* Trains evolve throughout the years based off of technology. Early on, coal-based trains are slow, clumsy and unreliable. Eventually diesel trains pop up but are unreliable until the technology matures (in contrast: coal/steam is at max reliability). Then electricity (but diesel is now max reliability, and electric is low reliable). Etc. etc.

* No "inserters", no "biters", no "defense". No blueprints, no factory designs. Its purely a game about moving items from one place to another. Turns out that in of itself is sufficient to make a game around.

* You cannot control the placement of primary industries or cities in OpenTTD. They're part of the "terrain" so to speak, its your job to connect them somehow.

* OpenTTD automatically grows factories / towns / industry based on how quickly you service them (aka: Trains grow things the fastest). The better you service a town, the more passengers it generates. As such, you INEVITABLY will grow until your network saturates.

* As such: good players aim for extremely high bandwidth designs, knowing full well that all aspects of the game will grow to meet whatever bandwidth you design. (In contrast: Factorio only needs to be designed for the level of traffic you ever plan for a region. In fact, mines begin to run out of ore eventually, so traffic slowly declines in any given situation, sans "infinite mining productivity science".) The growth-formulas for towns / industry seems a bit unintuitive at first: but know full well its all about encouraging you to make faster-and-faster train networks.

EDIT: * Latency matters in OpenTTD. All goods have a "rot" rate, so to speak. Things like Coal almost never lose value, but passengers and mail (two things you can transport) are extremely sensitive to latency. Running partially-empty trains for lower-latency is possibly more lucrative than running full-trains (but having your passengers and/or mail wait longer).