Under no circumstances will the SNCF replace any tickets even if they are lost or stolen. So, I suggest you equip yourself with a “waist purse”. A purse that is close to your body where you can keep all your precious papers (passport, driver’s licence, money and travel tickets).
A word to the wise, never separate yourself from your waist purse, and NEVER open it in front of or at the request of strangers. I hope you know what I mean.