Note: You HAVE to go to the Airtrain station and get the ticket from the kiosk. Once you have the ticket, you can ask an Amtrak agent to change to an earlier train (if there are any). As noted, they're all reserved now, so you can't just board a different train w/o permission. Once you're on a train, the Amtrak T&C (to which you're bound, not the CO terms) don't require that you remain on the train until the ticketed destination - just that you don't remain on past it.