in those parts of EUR it does not matter what is written on the side of trains/wagon: it is the one who owns the rails who determines the rights of the relevant state-company-or rather behemoth institute. So the very same train suddenly becomes from SZ to MAV when it crosses that border. As was/is the case with all INternat trains
AFAIK in summer there is-not daily-a holiday express, overnight from Bp via Zagreb to Split-with all-in fares and not taking passholders. Not to Rijeka/coast of SLO
There IS however a real private yellow RegioJet train od Prahy -via west of HU (not Bp) to the sunny coasts near Rijeka-OPatija, with also a portion to Split. This is thus also a little different from my 1st explan. It pays for traffic rights to pass. Will not take pax for shorter sectors and is often so late that it has to turn short of destination-pax bussed onward.
As to 1st: in fact this train is about the most reliable there is on Serb railways-except for the near inevitable delays next morning.