Last year I had a ticket on a certain TGV. I arrived let due to getting a coffee. I got on the next TGV, an hour later. I figured I'd have to buy a new ticket on board.
Surprisingly, the chef said, no, you're allowed to do that if you miss it, your previous ticket is good. I paid nothing at all.
Similarly, a friend reported he one got on the next-earlier TGV, and surprisingly agin did not have to pay. Another third anecdote was similar.
I'm surprised by this - in fact does a TGV (or similar) ticket allow you to take any train? Or is it for a specific time? Or were these just confusing cases of the chef "letting someone off"?
Surprisingly, I can't really find any official info on this, or even people blogging about it.