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Now you need ID even for a Greyhound or Amtrak.I've taken Amtrak quite a bit in the last few years (Emeryville & Truckee in California, and NYC, Philly, BWI, and DC in the east), and I've never been asked for ID to board a train. (In other words, trivially easy for someone to do something nefarious if they wanted to.)
No security checkpoints, no ticket checking before getting on the train, just someone checking tickets on the train itself 10-20 minutes after it gets moving.
You also don't actually need an ID to fly, though if you try that, give yourself plenty of time to get through "extra security".