This summer, pretty much any credit card I throw at SJ's website gets rejected now--regardless if I try in the US or Sweden. Only by pure luck does a transaction go through after 10-15 attempts. This also happened when trying to buy bus tickets (bus4you.se) and amusement part tickets (liseberg.se). I also confirmed with the credit card companies (Citi Costco Visa and Chase United Mileage Plus Explorer Visa) that they never received a transaction request. It's just easier to ask my father-in-law to buy the tickets, and we Swish him the money.

The only commonality I've seen is that these sites use paynova.com as the clearinghouse. If I had the energy, I would register a complaint with them and ask them to figure out why US-based credit cards are not consistently accepted.