The French rail system (SNCF) is extensive and very efficient. We often use it to get from Paris to the beginning of our walk and back again at the end.
The TGV (high-speed train) network is more expensive than normal trains (Intercités and TERs), but saves time and is quite an experience.
To book a train ticket on the SNCF network, go to the SNCF website.
When buying your tickets you are given the choice of an e-ticket (to print at home or to save on your mobile), or a hard copy ticket sent to you by mail.
Tickets go on sale about three months before travel, and there are a limited number of cheaper tickets available on each train, so book early.
If the tickets for your train are not yet on sale, you can subscribe to receive an email notification when they become available.
For regional trains (TERs), no reservation is necessary or even possible. The train can never sell out, and the fare is fixed so there’s no advantage in pre-booking. You can even take another train on the same day before or after the initially chosen journey, provided the route and the price conditions are the same as the ticket that you bought.
An oddity of the French rail system is that tickets (but not etickets) must be “composted” before boarding the train.
This means getting them punched in a little yellow machine at the entrance of the platform.
Tickets are always checked by a conductor on the train and if they are not composted, you will be frowned upon, or even fined.