Within the community it's a bit of an in-joke. It's not a coded message or anything, just an acknowledgement that autistic people are disproportionately into trains.

But they're just so cool! How is everyone not into trains this much?

Strictly my anecdotal observation but, as someone who attends train shows regularly, they definitely, absolutely are.

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.