My first thought was is (1) more of a projection of (3) from multiple dimensions to one, or more like the magnitude.
Also, it is known thing or are "trains" a euphemism now like "friend of Dorothy"?
I don't think it's quite the same as calling yourself or someone else a "friend of Dorothy". People who say they are into trains usually precisely mean they are into trains.
Not an ounce of complaint to be clear. Honestly seeing them flip out and flap around and giggle excitedly is delightful. I'm glad they're having a good time and I'm also glad that all of these experiences have not involved some self-involved asshole leering, criticizing or yelling at them for being happy.
I meant it only as a reference that one of the common characteristic symptoms of autism is a deep focus on some topic of special interest. In boys with autism, trains, cars, or other machines are a common one.