Book this one on cd.cz : way more convenient and no booking fee. There are usually better and cheaper ways to book than interrail.eu

- 454 CZK (19€) for 2 couchettes

- 1452 CZK (61€) for a private double sleeper

You're lucky : it's a cheap night train and it has availability !

Seat reservations and the app are completely separate. They never appear on the app, even those made on interrail.eu.

Whenever asked you show both the PDF (reservation) and the Rail Planner app (pass) with the journey added in order to get a QR-code, your ticket.

General advice : have a look at →

https://www.seat61.com/interrail-and-eurail-reservations.htm

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

The rail planner app is entirely separate from reservations. The app can't book any reservations and reservations you've booked do not show up there. In the app, you always have to add the trains you use yourself.

I’m not sure where the rail planner app mentions polrail.com. They can book indeed, but also have a booking fee and you'll have to pick the reservation up somewhere. This night train can be booked online on cd.cz without any extra fees and you'll get a PDF reservation; use "Reservation only” and choose the accommodation you want (seat/couchette/sleeper).

Edit: only the through seats are fully booked, couchettes and sleepers are available.

Also note that this train stops at 3 stations in Warsaw. 19:40 is the last stop in Warsaw but one of the others might be more convenient, especially the central station at 19:31.