But isn't that the case with any connection, Amtrak or not? Just last year this happened to me, flying RDU-PHL-HPN on US. At PHL they claimed I had not flown RDU-PHL and the system had cxld the second leg. I had flown. Evidence of a warm body standing in front of the gate agent at PHL was all it took to reinstate. With airlines it is hard not to scan the BP and still get an accurate passenger count. Yet this still happens, and they can reinstate reservations.

With Amtrak it is fairly easy for the conductor to not collect the ticket, and since there are no pax counts this will go through unnoticed quite easily. I suspect that if UA were to cancel a res based on a no-show on Amtrak, and make the cancelation stick, they would have a hard time to prove that this was a legitimate cancelation and not IDB.