How does the app work? Showing QR code for trains not requiring reservation sufficient?

Thanks a lot for your answer! You helped to understand better what I should do. 

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

@Fincholy as other people said, usually if you haven’t reserved a seat then you just show your pass QR code to the conductor. Sometimes they scan it or they just want you to scroll down to see that it’s valid today.

On trains which need both a reservation and an Interrail/Eurail pass, it varies. Sometimes the conductor doesn’t check the seat reservation (the seat is just reserved from one place to another) and they only want to see the ticket. In other places, they check both - for example on the Eurostar your reservation acts as a ticket, but you can’t get through the automatic check-in gates with it, you have to go to a real person and show them both your reservation and that your Interrail/Eurail ticket is valid today. They check your reservation in the computer and then let you through.

I love just hopping on a train. In the UK, for example, reservation is only compulsory on GNER routes. On other routes, including GWR, you can just hop on as they always have a few seats unreserved for people who turn up at the last minute. If you’re in 1st class this is great - you just collapse into an unreserved seat and then are treated like a king or queen as the staff come and offer you complimentary drinks and a meal. 😝

There are no day trains in the UK with compulsory reservations. If you mean LNER, reservation is very much recommended but not compulsory and they have limited unreserved coaches on all their trains.

Reservations are free in the UK and at busy times many trains have standing room only, even on long distances so why not reserve?  LNER even allow on the day reservations at the station up to 5 mins before boarding so if you have boarded early and taken an unoccupied seat in any carriage except C (the only unreservable coach) you could still be ejected from your seat before departure or at any station on the way.

Although I always print out copies of reservations, in the UK they are only used to guide you to your seat and in case you need to eject an unwilling occupant, so the confirmation e-mail would suffice.

An earlier post suggested you needed to get somebody to let you access Eurostar - not correct at Brussels or Lille - the bar code on your reservation will let you in on the auto gates. Your pass is likely to be checked onboard, along with your reservation.