Buon giorno.

 This Q has of course also been put here more as a few times.

1.most of these treni/stazione will have gates that you can only pass with the code on the RES. Even in Italia with TrenIt sometimes.

2.it depends-on where/when/etc-as about all thing in life do. But you risk being seen as ´black rider´ =travel with no valid ticket and can be fined-up to 200€ sometimes and thrown off the train next stop.

For trains with mandatory reservations you need a reservation. What happens if you don't have a reservation varies. In the worst case you get a heavy fine. Some trains you can't even board without a reservation. 

Quali treni vorresti prendere senza la prenotazione obbligatoria? Forse ci sono altri treni nella stessa tratta dove non serve la prenotazione.

Hello! Thanks for your question. I really would advise you not to, as you risk a fine! 

I hope this clarifies it.

Kind regards,

Annie

Hello, my question is if a can make the seat reservation at the train station ? I’m travelling long distances, so I would prefer to reserve the seat once on the station. I don’t get the part that if I don’t reserve, I cannot travel. This is my first time in Europe, is it really true that the trains are fully booked ? I’m specially concern about a train from Lyon to Lile. 

The vast majority of trains don't require seat reservations but high-speed trains in Italy, France and Spain do (+ some cross-border trains). TGVs can be fully booked a few days in advance, especially the ones avoiding Paris like Lyon - Lille as there aren't a lot of them. These reservations can be made at the station but they may well be sold out (unfortunately).

What is your travel date ?

TGV seat reservations have a limited 10€ price, then it's 20€ until the train is full. Thalys and Eurostars have quotas and these trains can sell out weeks in advance.

If you want advice, let us know your route and travel date.

Great thanks for your answer. 

My travel date is 28th April, If I can make it from where I am at the moment ( bourg Saint Maurice) to Lyon the same day. I’m finding the same problem, no seats available according to the Rail Planner App, but there are seats if I just find the route with other webpage. 

The rail planner app can't book any seats. It just forwards to some of the websites where you can book, including the Interrail website, which is not the most reliable one.

If you're in France, just go to the station, that's also cheapest. The ticket office and the ticket machines can sell pass reservations. Add Interrail as a discount. There is still a lot of availability from Lyon to Lille, some trains even still for €10.

For pass holders this is a dedicated site for domestic TGVs.

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish