I didn't really read past the first few posts in this thread. Just a disclaimer.
I have no idea how to, and I don't really know how anyone would quantify 'traveling' folk in the US.
When it comes to train-riders, though, I was talking to an older, more experienced friend of mine a few years ago. I asked how many riders he thought were out there at the moment, and he asked whether I meant how many stupid crust lords are stumbling around train yards once in a while, or how many actual train riders there were. I asked for what he'd call an 'real' train rider, and he said pretty simply that he'd put that at someone who rides more than 50 trains a year. He put that number about 500, at an obviously rough estimate. Which sounds like it's probably more accurate than people would think/like to think. There's a lotta folks around claiming to ride fr8 hella hard bro, who barely touch the rails in a given year.
Just Food4Less. Er, Thought.