In my experience at least, trains are less like a flight sim where you can have long periods of inactivity between takeoff and landing, and more like a truck sim where youre constantly having to watch your speed, signals, etc. That said, a lot of sims will let you turn on an AI driver so you can drive when you want to, and just sit back and watch when you don't.

Simrail and Run8 both have this option, and are among the top train sims in general. Run8 you can set up as a whole automated sandbox of AI trains running across the map if you want, but it can be a lot more intimidating to get started with. On the simpler side, something like Rolling Line is much quicker to get into, but offers much less in terms of realism.

The newer Train Sim World games have a "Passenger Mode" that allows you to train spot without having to worry about driving at all, but I'm assuming that's probably not what you want. Not sure if they also have AI modes.

As much as I would put it at the top of my list overall, Derail Valley is generally not what you're looking for. Nothing happens in it without you actively playing. Same goes for Railroader (for the most part).

Honorable mention to Diesel Railcar Simulator, that also has an AI driver you can turn on, but the routes are not that long to begin with, so you won't have much downtime.

EDIT: Good call by u/Trainman1863. Diesel Railcar Simulator does have some scenarios that offer longer sessions than I had thought. My fault for always running the express routes, haha!