In the past summer, there weren't many issues. But currently, it's changing almost every day and nobody can tell you what the situation will be in a few weeks’ time, let alone in a few months’ time.

Use reopen.europe.eu as a starting point when your journey gets close.

As rian says, it can change by the day, if not minute and it really is completely impossible to tell now what will be the situation early next year.

Ive used an Interrail for 2 monthes in sep-nov and then it was relatively problemless, but that QR code (I really DK as you did brexit and moved away from all things EU how that works for british) was very often needed and in some countries there were also special rules re the mouthmask you have to wear plus one has to fill out an online entry form (often named PLF=passenger location). The site europe reopened has the best info re current regulations but again its mostly aimed at EU-citizens. Otherwise read a general worldwide touristy forum like tripadvisor or maybe that trusted old friend seat61

Note aside: an IR often is not that worthwhile to use in those countries as local fares are often cheaper. Especially if you fly in/out there.

Hi!

I have been on Interrail for two months with my wife, next week we will be flying from Berlin back home  to Finland. 

We've not had any problems at the borders. With the vaccination certificate everything has been easy also in restaurants, museums and so on.

Some countries require online notification (Italy, Malta). As previous writers noted, the situation can change quickly and many times in a short period of time.. I've read the pages of the countries' authorities and we have not met any surprises.