SNCF, France's national rail operator, has clear nationwide pricing for pets that's worth knowing before you book. On TGV INOUI and Intercités day trains, a small animal in a carrier or a dog on a leash costs a flat 10 euros per animal per trip, regardless of distance. Intercités night trains charge 19 euros per animal in seated cars, and animals aren't permitted in couchettes unless you book a private compartment, where the fare drops to zero. Guide and assistance dogs travel free throughout.
TER regional trains set their own fares region by region rather than following the national TGV rate — some regions charge as little as 5 euros flat, others closer to 7, so it's worth checking the specific region you're travelling through rather than assuming the TGV price applies. OUIGO, SNCF's budget high-speed service, also accepts pets under its own separate pricing, detailed on the OUIGO site.
Whatever the train, the physical rules are consistent: a small animal's carrier can't exceed 45 x 30 x 25cm, must be closed and ventilated, and travels at your feet rather than on the seat. A dog travelling on a leash rather than in a carrier must be muzzled for the entire journey, and other passengers' consent is technically required for any pet other than a guide or assistance dog. Only guide and assistance dogs are permitted on SNCF's rail-replacement buses.