Model trainers used to be 2 wires to the track from a DC transformer. Add in a switchboard if you want blocks. Set the locomotive on the track and it would run based on your output from the transformer.

Now they are mostly computerized, and prohibitively expensive. A decent steam locomotive used to be $300-500... Now in the $700 range. Granted there is new functionality, but it feels like a hobby simple enough for a 7-12 year old to enjoy became an old rich man's hobby.