Chase was the most recent issuer of the Amtrak Guest Rewards credit card, but the card is not currently available. If AGR ends their relationship with Chase they may well also leave Ultimate Rewards.
The current rumor is that Citibank will be the new Amtrak-affinity credit card in time for 2016. If so, I sure hope that ThankYou points (of which I now have a tidy stockpile) are easily transferrable.