Originally Posted by Louie_LI

Um, yes you do have to punch normal TGV tickets, unless it's international. Punching the ticket serves a purpose: it shows that the ticket has been used, in case the conductor doesn't come around to punch it, and prevents one from trying to get a refund for a trip one actually took.

If you ever run out of time or forget to punch your ticket, find the conductor before s/he makes the rounds. If you come clean up front, they are generally lenient.