I think kids are always excited by new stuff, and if it's big or complex stuff the excitement generally increases accordingly. Buses, trams, excavators, trains, planes, firetrucks, road rollers, cranes. And then as they get used to them the excitement either subsides, or it just keeps getting more and more specific.

So for a European kid who at some point starts traveling by train regularly, it either subsides quicker, or it has more of a chance to get specific, because they start noticing the differences between the trains they use, and possibly the tracks if they use multiple kinds. This eventually results in lots of train nerds among grown ups.

By the way, now I'm wondering, is the hobby of building and maintaining and running model trains on model tracks in a fixed installation at home common in the USA? I know at least three people who do that here in Switzerland, it's not like sports or something, but for the large effort it still seems relatively common to me.

One colleague at work has all the train models ever used by Rhätische Bahn in his collection now. That's a regional train company that only serves mountainous regions by way of narrow (1m) tracks in one corner of Switzerland, but it's still a big collection.