Having now worked out what the SNCF Voyageur program is, I've gone ahead and joined it, as I've a number of TGV journeys coming up in the coming months.
The slight wrinkle is that most of these TGV tickets have already been purchased, and none of them have my Voyageur membership number on them. (Some were booked before I joined, some were booked after but with sites that don't seem to support entering them).
With Eurostar, if you log into your Frequent Traveller account, then after travel you can enter the ticket details and get the credit for the travel. With most airlines, you can log into your frequent flyer account in advance, and attach your membership details to the booking.
Despite that, try as I might, I can't seem to find how to do one or the other when logged into my SNCF Voyageur account.
Is it possible to credit tickets booked without your SNCF Voyageur membership number, but travelled after joining, to your Voyageur account? And if so, how do you do it?