Personally if I were going to adopt a nerdy train hobby, I would tend more toward train photography. Recently train photographers have been in the news for mostly bad reasons [1], but I have also seen train photographers setting up in rural locations and the scenery looks stunning and also totally chill. The problems arise when people gather en masse to get the "iconic" shots that have been probably been photographed a million times before.
Or just go out and actually ride a bunch of different routes. It's been a long time since I've done it, but just riding a local or express train through a scenic area is delightful.
Of course there's no reason that true train afficianados can't do all of the above, as well as building model trains!
[1] https://petapixel.com/2025/12/15/japanese-railway-pleads-wit...