It took me 6 months and 20+ emails to get the refund back to my credit card.
First I was offered refund (more correctly, compensation) by either bank transfer or credit card. I didn't want to provide my credit card number so I chose bank transfer.

Because of some banking system error/issue/whatever, it couldn't be done.
The main agitation was that Thalys replied to my emails 1-2 WEEKS after I sent it. It took 2 months to find out bank transfer couldn't be done, then 2 months of hearing nothing (I was emailing once a week!), then finally 2 months later the refund showed up in my CC statement.

I made sure I was emailing to the same representative (a lady) so she knew what was going on. The email address was a generic info/customerservice@thalys I think, but I always addressed to that lady.

Terrible.