Esteban European type cards with 3D-Secure authentication or American type card without?
This may play a role because the bank and payment processor used by russiantrain.com is Belarusian (bepaid.by) so they use 3D-Secure gateway.
But then another reason may be that the card's bank does blacklist Belarus. I have the case, one of my bank keeps Belarus, the other bank not.
Alternatively, on the payment window of BePaid, the one where you fill in card's data, Google Pay is offered.. This may work or not (depending if their G-Pay account is the same bank or not).
you could also try tutu.ru, it worked for a while, don't know if their workaround was hit by gringos since.