Edited later to add: (apologies for all the bold but many people are not reading or understanding what's below)...
There is a LOT of misinformation posted in this thread.
This is NOT about your bank suspecting fraud and declining a credit card transaction.
You can NOT fix this by contacting your bank and alerting them that you will be making foreign transactions.
This has nothing to do with your bank.
Your American credit card is incompatible with the system being used for the transaction.
Please stop replying with suggestions to call your credit card issuer.
If you don't want to believe me, fine. Here's what Rick Steves staffer Laura said about this, in response to my original post on the problem back in January:
The 3-D Secure standards are a common problem for Americans trying to make online purchases on those foreign web sites that have chosen to implement the procedure, including many in France and many railways (France SNCF, Norway NSB, Austria OEBB, etc.). " Short-circuited" is a good description - if it doesn't detect that you're in the program, it does just stop the whole approval process.
You can read more of her response (and more about this problematic system) in my original post from january here:
3-D Secure credit card payments - wha???
You may have run into this new (and highly problematic for Americans) "3-D Secure" system. In short, it's a new system that's increasingly common in Europe, used for online purchases, requires a credit card which is tied to a mobile phone account so they can send you a verification code to complete the transaction.
I ran into this back in January and was hitting one brick wall after another. The vendor insisted my credit card was being declined (I tried many different cards), my credit card issuers assured me there were no declined transactions and that the vendor was not even attempting any charges. Round and round it went for a couple days until I discovered there's this new "security" layer added - and American credit cards do not work with it. I was unable to complete the transaction and had to end up wiring money.
It seems this is an increasingly common problem for Americans. American banks don't seem to know anything about it (mine didn't - major US credit card issuers eg Chase, Citi, American Express). I guess you can add Capital One to that list.
I believe it's commonly used for processing the purchase of train tickets...so I think that's what you ran into.
AFAIK there's no way to make it work. You need a credit card that is "3-D Secure" enabled, and I believe no American credit cards are (or plan to be).
Here's the thread I started in January when I stumbled into this mess: 3-D Secure credit card payments - wha???
Good luck.