I don't know the answer to your question. However I would think that it would be cash or CC. Particularly since your sea pass card is linked to the ships "computer" payment system, for onboard purchases. I don't think that would transfer over to the train. I could be wrong though. I would contact RCCL directly in order to make sure. There is a "Contact us" email system on their web site, or you could just call directly. If you have a TA, have him/her find out for you, they can call for questions like that probably faster.
Hope this helps
Independance of the Seas
Monarch of the Seas, October, 2007
Serenade of the Seas June, 2006
Three years ago we took the train on our Celebrity land tour. Royal and Celebrity use the same trains. We used travelers checks or cash for our meals--breakfast the first day from Anchorage to Talkeetna and lunch the second day from Talkeetna to Denali. We spent two nights in Denali. Third travel day we were on a motorcoach for a shorter trip that didn't involve a meal and ate lunch in beautiful downtown Fairbanks. We didn't have our SeaPasses yet, but those are only for the ship anyway.
We had all our paperwork finished before we left Fairbanks and were issued our SeaPasses on the coach in the Seward pier parking lot before we boarded the ship.