is it cheaper to get your train ticket for a French route in the US before you leave or is it the same buying it there?
Booking TGVs and certain other trains well in advance (up to three months allowed) can get you discount fares, but there are some tricks you need to know. If you give me your route(s), I'll give you a full explanation.
There is no discount for booking regional trains in advance. The standard fare is your only choice.