I did not need to show ID to hop on the brewster line into grand central.

Because those are public roads. Not high priority roads you pay for access to. Just like fully government funded buses and the like don't care if you have a ticket or not. Whereas those that are only subsidized tend to still require a pass/ticket/payment.

Also I'll just point out: Your car has a license plate. It is trivial to read those with an overhead camera. And, depending on the road/day, you can bet a cop will gladly run you down if they spot you have no license plate or decide they care about the polarized screen you put on top of it. At which point you can bet they'll want to see your ID and won't take "I am a sovereign citizen and this is a public road" as an answer.

there is nothing special about flying compared to trains

And you need to tie your body to your ticket when you get on a train. We tend to not care anywhere near as much about security with trains (because everyone knows bombs and box cutters only work if you are at least 100 feet above the ground!), but you can be damned sure the ticket machine where you inserted your cash took a picture of your face to go with it.

I genuinely don't care about what kind of libertarian nonsense you are on about hating the concept of IDs at all. My point was simply that if you think the RealID requirement has ANY bearing on The Man knowing where you are and where you are going then... you don't actually understand what your exposure is.