Not sure if Portugal is the type of European country you imagine, but last month on a train I sat one row ahead of a child that would excidetly exclaim "another train!" (in Portuguese) every time one passed outside the window. Adorable!

... given that they are super common?

Oh man, I think it's the other way around. In Japan, the country with the highest rate of passenger train usage in the world, rail fans are a well-known category of nerd.

At one station, there was a mini museum and display of children's train artwork. Saw kids proudly posing in front of it for their parents to take pictures plenty.

Tractors are ultra common and yet kids go bonkers for them

Trains, Traktors, Excavators, Bicycles, streetcars, wheel loaders, trucks, dump trucks, street sweepers, garbage trucks...

Regardless of how common those are, my toddlers will loose their shit every single time.

There is always a first train and the same first reaction

Yes, my 4yo will point it out excitedly every time he sees a train.

Croatian here, and yes (at least speaking for myself).

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.

Check this out: Children's railway.