American credit cards on NSB website

NSB uses 3-D secure. Try registering your card with Visa Check-Out or Verified-By-Visa. Some have also had good luck with American Express.
You should read this page: https://www.nsb.no/en/our-tickets/buying-your-nsb-ticket

You should also let your credit card issuer know you are tying to do this. Most US cards are blocked from any and all foreign transactions unless you notify them.

https://www.nsb.no/en/our-tickets/buying-your-nsb-ticket

Online

You can book your train journey using the Travel Planner. Should you
experience problems using your debit or credit card, please contact
the NSB Call Centre , phone number 815 00 888/+47 23 62 00 00. Press 9
for information in English.

We are continuously working on establishing secure payment for our
customers. If you are having issues with payment, please contact our
customer service or try booking at scandinavianrail.com .

My card is registered with Verified by Visa. Believe me, I've read the link, and everything I could find on making this work. I'm just about ready to book through Fjord Tours and skip the hassle of trying to save $100. And I was able to use the same card to purchase -- hassle-free -- the ferry portion of NIN (which is refundable).

I am wondering if this has to do with rewards features of American credit cards. European regulations, recently enacted, prevents charging of more than a very small percentage for credit card transactions, somewhere in the 0.5% range. Rewards cards, which have to charge higher fees to pay for the rewards, may be rejected off hand by the merchant because of the higher fees. Restaurants, hotels, retail might put up with the higher fees because they already operate with a healthy margin and can stand paying an extra 2% because the added business makes it worth while. But public transport entities, which are probably already operating somewhat with taxpayer-supported funds, are not prepared to take any such loss.

Have you tried calling them on the phone number provided?

I haven't tried calling yet. It seems like the NSB website works for some people and doesn't work for others. Another complaint I hear is that you can book one trip with your credit card, but then if you try to book another trip or leg of a journey, they prevent you from using the same card. The card I am trying to use is the Andrews Federal Credit Union Visa, which doesn't charge foreign transaction fees. I'm about to conclude it's not worth the bother to book this myself to save a few bucks.

It is not whether they charge you fees, but how much they charge the merchant fees. I also have an AFCU Pin & Chip credit card and it is a rewards card.

I reserved two summers ago for two NSB tickets. I had exactly the same issue. When I called I was told that you can't use American credit cards online to book the tickets so had to reserve with an agent over the phone. It was no problem though!

We also have an Andrews card and honestly it is more trouble than it is worth. We will bring it on this next trip as a backup card and then we are giving it the heave ho. We also had the problem you are having. Called the card co and of course you get transferred over and over again. They outsource everything, so you keep getting transferred for every little thing. Then it still didn't work, had to call again. In the meantime they blocked the card, period. They claimed to have tried to call us, but of course they didn't. It's annoying.

We usually use a capital one card, same benefits without the hassles. If there is a questionable transaction, they simply send you an email asking if it's a valid purchase, you answer yes, and you are good to go. Of course, we always call to let them know where we are traveling.

Last week I was able to book NSB after much angst. No, my CC was not blocked by my bank, yes, I had Visa Verified. I called NSB & they said try tomorrow. I used Internet Explorer then switched to Chrome. I also noticed NB site advises they take PayPal. I updated my CC (as other had lapsed) and tried my PayPal with Visa on Chrome and on Internet Explorer. No luck. Finally I did an online chat and the NB online chat person went through all the issues I noted above and as a last sentence before we were through she said try Firefox. I did. I was successful! Wow! Such relief and at any point would have been easy to just throw up my hands and give up. Persistence paid off! I have now, but for the bus tickets as all say buy on the bus, booked and paid for two self booked Norway in a Nutshell tours for my husband and myself for 5/24/17.
We are traveling from Oslo following NIN leaving at 6:25 am, using the fast ferry and arriving in Bergen about 7 pm. We stay over in Bergen for 3 nights. I'll do a trip report upon our return. I would not have had the nerve or the knowledge to book without the insight and advice from this forum. Thanks!