My friends & I all were touring on folders (Bike Friday). While Bike Fridays are designed to pack securely into a suitcase, they're not really a quick-fold. We would have gladly paid 10€ each to check them, but the limited bike slots were all already reserved when we bought our tickets. We brought large plastic trash bags to use if we were unable to check them. We removed the pedals, folded them, and put 'em in the trash bags. They're still unwieldy, however. It's a bit unnerving boarding the train because lots of folks were bringing large suitcases into our car and the baggage area is usually insufficient on crowded trains. 2 of us quickly got our bikes on board while the 3rd dealt with our panniers which could go in the overhead racks which weren't full. On a previous trip, we were connecting from a TER train (a slower, regional French train) where we could hang our bikes ourselves (for free) on hooks, to a TGV. The baggage people told me there was insufficient time to check them on the TGV during our connection, so we used the trash bag option that time, too. I got the idea of using large trash bags for a French cyclist. BTW, on that TER train, more cyclists boarded at a subsequent stop after we had boarded and there were no remaining hooks. The cyclists jammed them into the area between the railcars and the ticket inspectors didn't complain.
Here's some more French train experience which might prove useful to someone in the future. A friend & I were trying to take a train back to Paris on a day when SNCF (French railroad) workers had announced a strike (somewhat amusingly called a mouvement social). Alas, strikes are not rare in France. I've also dealt with railstrikes in Italy. There was going to be only one train going to Paris from the town instead of perhaps 10 daily. The ticket office had reduced hours and they weren't permitted to sell us tickets for travel that day, but they gave us a document which would prove to the on board ticket inspector that we had attempted to purchase tickets before boarding. That way, we wouldn't have to pay the penalty for buying the ticket on the train. Strangely, that lone train we were able to take wasn't crowded (I had feared a cattle car), and no ticket taker/seller ever appeared so we rode for free that day.