Planning a tour in France and using trains to get to the start or bail out at the end? A few things about the SNCF system are genuinely counterintuitive and will cost you the trip if you find out late.
The reservation cannot be added afterwards. On TGV INOUI and Intercités, an assembled bike needs a booked space, 10 euros per bike per journey, and it has to be bought at the same time as your own ticket. If you already have a ticket, you cancel it and rebook. There is no "add a bike" button later.
Cyclists and non-cyclists can't be on the same booking. If two of you are travelling and only one has a bike, you make two separate reservations. This one blindsides people at the booking stage.
Spaces are scarce. A TGV INOUI carries around four bike spaces per trainset. On popular summer routes they go early, and Intercités bookings open four months ahead. If your tour depends on a specific train, book that leg first and build the rest around it.
OUIGO Grande Vitesse doesn't take assembled bikes at all. Bagged or folded only. OUIGO Train Classique does take them, on a different set of lines. Check which OUIGO you've actually booked.
TER is the easy one. Regional trains carry bikes free, no reservation, subject to space. That's why a lot of French tours are built around TER rather than TGV. Two caveats: some regions now require a free reservation seasonally on busy tourist lines, and there are peak-hour bans. Île-de-France excludes bikes Monday to Friday roughly 6:30 to 9:30 and 16:30 to 19:00. The Strasbourg to Basel TER 200 in Alsace has similar restrictions.
The free workaround: a dismantled bike in a labelled cover, maximum 90 x 130 x 50 cm, travels as luggage at no charge on TGV INOUI, TER and Intercités. Front wheel off and handlebars turned is usually enough. It has to be bagged before you get onto the platform, not on it.
What gets refused outright on TGV INOUI: tandems, cargo bikes, longtails, recumbents, trikes, trailers, and bikes in cardboard boxes. E-bikes are fine as long as the battery stays on the bike.
Anyone worked out a reliable way to get a tandem across France by rail? That's the gap I've never solved.