Hey,

you can book only a reservation via oebb.at using One Way Ticket option (NOT ONLY A RESERVATION) and choose Interrail/Eurail as discount. So you get only the reservation for Nightjet. They start from 14 EUR for a seat. Couchette and sleeper more.

The price of €42.80 is correct for a couchette reservation in a 4-person compartment.

In the app you only need to register the train, shortly before you board.

Ok, thanks for the info. Had not in mind how much it costs. 

Thank you both for your help. I have also just followed Seewulf’s excellent step-by-step instructions on this!

This is where I have now got to - think this means I have seat reservations + a couchette booked for me and my family of 4. I believe this means that I do not have to separately book reservations on the eurail app.

Yes that’s it ! You’ve got a private couchette compartment for the 4 of you

You simply add the journey to the app before travelling, nothing else

Yes, the app does not book reservations. It linkes to the reservation service of Eurail/Interrail, but it works like a diary. You need to add every trip on the app to get your Ticket. Your Interrail Pass on the app is your Ticket.

Just to be sure - You can add (or remove) the planned trains to your trip at anytime and then before boarding your first train you activate your pass and trigger the toggle that appears by that train in your trip. That creates a QR code and shows the train details below the QR code. That is now your valid ticket that accompanies your evidence of reservation. As you begin journeys on further trains you follow the same procedure to transfer from your trip to your pass.