What type of pass do you have?

What train, date and time, are you trying to book?

From Amsterdam to Berlin, don't book via Interrail. It's cheaper via DB (€4.50 per journey) or ÖBB (€3 per train).

As such the whole trip can be done without RES-its only optional. For the direct IC-trains, running ev 2 hrs, this is advisable to do (see above) as you do run a real risk having to stand-part of the way (certainly not the full 6 hrs) on the usual busy days.

On MANY days to come there are works on various parts on the way and this may mean the normal trip is curtailed and you have to use other (local/domestic) trains for that part-these cannot be reserved and that leaves you to the very logical above. But it may only be the very last 2 stops IN Berlin at the end-in the case you are determined to see this giant main staton and not use metro to HTL at some other point.

Without exact details it is impossible to tell what is.