You cannot "check in” with an Interrail or any other ticket that only has a barcode. The barcode just opens any gate that you encounter.

If you have a paper pass, then there's a barcode on the pass cover that will open any gates.

With a mobile pass, the barcode (under My Pass > Show ticket) will open any gates in the Netherlands.. the barcode is only available after you've added at least one journey to your pass for the travel day. Do make sure you're using a gate that has a barcode reader. Gates that only have a chipcard reader cannot read barcodes, so you can’t open them.

If all else fails, ask staff or push on the blue information button that should be present next to each group of gates (usually next to the wide gate), say you have an international ticket but the gates won't open. A gate will then be opened remotely.

Okay thanks, so I can check in any gates with my interrail mobile pass, with the barcode? I only have to purchase my first train to Berlin in the interrail app then right? That is how it kinda works?

You can open any gates in the Netherlands with the barcode. This isn't “checking in” though. It's just opening gates; they will say "Passage OK”.

You'll need to add a journey to My Trip and then to My Pass and then you can show the barcode.

For the train to Berlin, you don't need to purchase anything if you already have your Interrail. Your Interrail is your ticket. You can book an optional seat, e.g. for 4€ via Deutsche Bahn (Interrail asks 8€ for this reservation).

Okay thanks! Yes the reservations..are they always needed? And can I see on which platform I have to go in the interrail app when I have a train change

Reservations are not always needed, it largely depends on the country. In Germany, they're optional. Check the country information here.

The rail planner app doesn't have platforms. The DB website or DB Navigator app show them and are also better in general. They're more reliable (2 or 3 updates/week instead of just one every 2/3 weeks) and also have realtime info for Germany and a number of other countries.

Thanks! That sucks.. so I can’t see where I have to go while changing train? Or are there a board at each train station where I can see

There are boards or atleast oldstyle timetables that show you on which track the train departs. Some Stations like Prague have no sheduled track for trains and the track will be usually announced 15-20minutes in advance via the depature board (like the gates on an Airport)

In Germany, just use the DB Navigator app. The Interrail app is the last source you should use for timetable information, since it is offline, is only updated every few weeks and is therefore not reliable. DB has 2 or 3 updates per week and has realtime information. National planners and apps are always more reliable.