Observation...cover the bike and you should be good to go. Also make sure you can handle your baggage without assistance. I ride Amtrak regularly, however I have not done it with a folding bike. The only two runs I have taken a bike on have been between Fayeteville, NC and Boston. I shipped the bike ahead of time. The other route has been Fayetteville to Charleston and/or Savannah, and return. On all of those trips I boxed the bike and it traveled on the same train I did. I have seen two folders on recent trips, one between Richmond and Charleston and another between Philly and DC. One was in a Dahon case, the other was a Brompton in a soft bag/cover.
Aaron