Got frustrated trying to get SNCF's website to take my US Chase Visa; called Chase and they said it wasn't an issue at their end, that it was SNCF that was declining the card for some reason.
So I tried RailEurope... uh, nope, very expensive. Then I remembered something someone mentioned here.
https://www.capitainetrain.com/tickets
Worked like a charm! Same low price as direct from SNCF, but no issue taking my credit card.
Hope this helps someone-