I got this message from SNCF this morning. CDC cards no longer accepted on TGV.
Regarding your request about the USA vaccine validity in Europe, I am afraid to inform you that they are not recognized if you’re not able to activate your health pass QR Code.
In this case, if you wish to board a long-distance train (TGV IC and international trains except Eurostar) you should have a PCR test done until 72h before the departure.