I just took a TGV train from Lyon Part Dieu to Paris. I made a reservation and sat with comfort for the 2 hours journey. However, I am surprised to see that many people are either standing on the train, where some even sat on the stairs leading to the 2nd deck.

It was very obvious that these travellers did not make any reservations on the TGV. As such, i was wondering if reservations are truly necessary on the TGV?

Well, apparently they are if you want to have a seat.

When all the seats are booked, the SNCF sells a few extra places without a guaranteed seat. In practice, some people book seats then miss the train or change their plans, so there are often odd empty seats.
If you board a TGV without any reservation, they will not stop the train to throw you off, but the ticket inspector, I believe, will charge extra.

Hi S,

Not only necessary, but required.

As GH says, the SRO tickets cost extra.

I was told that you could jump on a TGV without a ticket, but that you must go find the ticket agent before he/she finds you and pay the extra fare. If the agent finds you first, you will also pay a large fine.

That's an unusual experience, I have never seen people standing on TGV trains on any I've ever taken. Well, I've seen people standing for a bit in some areas because they wanted to but they also had a regular seat.

Yes, the official SNCF "rule" is that you must purchase a ticket at least 5 minutes before the TGV departs. If you justs climb on board you are subject to a fine.

I've never seen anyone just standing on a TGV who didn't also have an assigned seat.