I have the exact same problem. I don't remember setting up a password for this. It seems have only numbers... What to do?

Ah, found an answer for my problem!

1. At Step 3 Activate Pass, enter the code you use to UNLOCK your mobile. (Yes, it is that simple, but not stated anywhere obvious or logical!).

Ah, found an answer for my problem!

1. At Step 3 Activate Pass, enter the code you use to UNLOCK your mobile. (Yes, it is that simple, but not stated anywhere obvious or logical!).

Here's the information from the Interrail site about the password for the app.

https://www.interrail.eu/en/support/already-bought-pass/interrail-mobile-pass-authentication