Since we have another report of stolen luggage, I'll share what I do to try to keep mine safe.
I ride 1st class SNCF because only first class TGV shows you a seating chart and allows you to choose your train car and seat. In second class you can make a request, but the computer assigns the seat.
If sitting downstairs, I choose a train car with a luggage rack towards the back of the car. All lower decks in double-decker trains are deadends. Nobody can walk through the car and out the other end. I put my carry-on in the rack further back, not near the door. If I have a large suitcase, it will go near the door on the bottom. Those are cumbersome for thieves.
Or, I choose to sit upstairs because thieves are less likely to carry a suitcase down the stairs and out the door. There are usually more racks upstairs.
You can do a dummy booking to see where the luggage racks are located.
What do you do?