There are also trains to Split. What's your travel date?

I don´t have a definite travel date, but I wanna go in july/august next year. I only saw trains to Zagreb, with an 8 hour overlay there, which I am not a big fan of. Is there another way to Split? (through Serbia  maybe?)

No, the ONLY trainline overthere is from Zagreb. In SUMMER (exact dates not yet announced) is also an overnite train on the line. In old Yugoslavija are just very few trains east-west via the mountains. In the distant past, when it was still YU I once came from Beograd overnight to Ploce, via Sarajevo then bus along coast, but this is also only memory now. There will also still be BUSes along the coast SLO-HR. The former Jadrolinija boatline along coast/isles is long gone.

Also in summer there is a weekly CZ yellow RegioJet train from Praha, overnight, quite long trip, aimed at Czech holidaymakers. This train is NOT incl. in IR, but fares are very reasonable.

IN Hrvatska=Croatia is also a much more extended BUSnetwork, not just this green FLIX, in fact many more Croatians travel by bus, also longer distance, as by train. Google Zagreb Bus Station=Autobusni Kolodvor and see what it offers. Again: in summer there will be much more to the coast

Ah, I knew there weren't much trains, but reading it is the only trainline is a bit of a bummer. I might look into the Praha train and the buses in Croatia. And I'll definitely look (and wait) for the summer schedules of the buses/trains. Thanks all! ^^

In summer, there should be night trains to Split from Vienna/Bratislava, Budapest, Praha (RegioJet, not included in Interrail) and indeed the domestic night train from Zagreb. There may also be a second day train. Check the timetables again in May/June.