We travelled from UK to Croatia.  For the journey through France, Germany, Austria and Slovenia the interrail app allowed us to find trains for which we did not need to buy seat reservations.  However, all the Croatian trains said we needed reservations.  The information on the Interrail site leads you to believe that you can buy these through the OBB Austrian train reservation site.  This isn’t true. I wasted many hours trying to find some way of getting the reservations, sometime getting as far as the payment page, only to be thrown out with an “error” at the final point of booking.  At Salzburg and Ljubljana stations we asked if we could buy the seat reservations there and were told “only in Croatia”.  We were due to change trains in Zagreb with 4 hours between trains so decided to try there.  It was quite worrying as on the return journey we wanted a sleeper on the Zagreb to Munich overnight train, which would then connect to Frankfurt and the Eurostar.  So any problems would have led to us missing our Eurostar train.  Anyway, our train to Zagreb was 3 hours 50 mins late so we only had time to scramble to find our train.  The guard stopped us getting on the train demanding to see our reservations.  We explained and he let us on, telling us we may have to move seats if we sat in one allocated to someone else.  Well, the train wasn’t full and there were no issues. There was nothing on any of the seats to say if they had been reserved or not and we didn’t see anyone else being asked to move.

In Split we were finally able to book the reservations for the return journey with absolutely no problem.  We were able to book a sleeper, even though by then it was only 6 days until we got the Split to Zagreb, and Zagreb to Munich overnight train. However, we were travelling in November so goodness knows how this system operates in summer.  

There is definitely something wrong here.  From reading up about it, it would seem that the Croatian railways site www.hzpp.hr is not compatible with any other train booking systems. But the Interrail app doesn’t tell you that!  This may change when Croatia joins the EU in Jan 2023, but who knows?

My advice would be, if you have an Interrail pass that allows x days in a month/2 months, to buy  tickets for Croatia separately through the Croatian railways site, as you can get the reservations at the same time by doing that. The tickets are quite cheap and you will save a lot of headaches.