Hi Alizee,

Where there’s no reservations required, you run the risk of not being able to find a seat, or not being able to sit next to each other in a group. If this bothers you it is always possible to make optional reservations for IC (but not RE) trains in most countries. In Britain it’s free to do this at stations, so a total no-brainer. However, most of the time (at least outside peak commuting hours) you’ll be absolutely fine just hopping on and finding your own seats, and there’ll be plenty available. If you are unlucky, you will have to stand or sit on the floor in the area by the doors, but sooner or later a seat will become free again and you’ll get to sit down.

Okay thanks a lot for your answer ! And how we do to reserve a seat near each other ? There is a system in the interrail’s reservation or we have to ask to people in the train ? 

Provided the same person books all the seats (or they’re booked by your group together at the station), staff will try to seat you together as long as it’s still physically possible to do so. Through Interrail themselves I’m less sure, as I haven’t personally used their reservations tool myself and I don’t know how well it handles multi-person bookings. I am certain someone else on this forum will know though!

Okay, thanks you very much :)