I got interested when i when to toys 'R' us a long time ago and got my first train set
I wonder if it's genetic. My wife's brothers, Bob and Tom are hall-of-fame level railfans. If there's a train to discuss, they are like encyclopedias - both of them are. They are also live steamers, and Bob has a 7" gauge (1.5" scale) layout in his back yard. If you look up railfan in the dictionary you'll see their pictures.
Besides these two brothers being train guys, Bob's son is also a train guy. His is a 1.5" scale "diesel." It's an F unit that runs on gasoline. Two of Bob's grandsons are also showing interest in trains. This is starting to look more inherited than learned.
Myself, I'm more of a modeler than a train nut (so is my father, although he likes model airplanes). I like to look at trains with an eye toward building a respectable model, but I'm not into trains themselves like Bob and Tom. They're loving it if they can go hang out by the track and watch the trains; I'd be bored silly. My thing is to look at other model work - I am really fascinated by the approaches that other modelers take to get good results. At a train show, I'll be there all day - I take it all in.
Based on all of this, I think that the train gene is one thing and the model builder gene is another, and that the susceptibility to become either a railfan or model railroader is hard wired (we're born with it).
Phil, I'm not a rocket scientist; they are my students.