Which country / trains? 

Hello, I realize that this answer is too late to be useful but yes we do advise that all reservation e-tickets should be printed and available for inspection along with the QR-code on your app or filled out paper pass :) 

However, except for (some?) night trains, I'm not aware of any trains for which this is necessary. The fact that this information provided by Interrail does not correspond with reality, makes me and many other just ignore it. Possibly also for trains where it is necessary.

I agree with @rvdborgt that showing the reservation on your phone works fine. Your answer will stress people and make them searching for printout possibilities all over Europe when there is no need.

There has been questions from travellers who plan to cancel their trip just because they don't manage to find somewhere to print the reservation. 

The only case where a printed reservation might be good is an international night train with a ticket check in the middle of the night. The you can give your reservation to the train staff from the first country. But if you travel with a mobile pass you anyway might have to be woken to show you ticket.