You need to check the timetables of the national railways of those countries. Check also information on seat61.com 

As for now and some time to come Serbije ONLY has INTERN trains to Black Mountains=Cerna Gora. NOT as used to be and one can perhaps still read in older guidebooks, to HU or BG or Macedonia. That will be bus. Also applies to these other small former Yugoslavija republics. Plus that there has been hardly any upkeep of raillines, so they fall in disrepair and so do the even possible services.

ON 12/12/22 all rail timetables changed-a very little or more and these have (just think, first law of computers) to be loaded into a kind of central hold-system-named MERITS, and only then can be reloaded into other devices. State run burocracies like most railways are often very late with that-you wait, thats not their prob. OR you check the sites of the railways themselves-it should be plain obvious these are the first to have new data. This will apply to ZSSR for SK. Very young or senior people can get a pass locally to use local trains for free in SK. Bring passp foto-this even still applies to those from out of EU.

the site seat61.com is best for uptodate general info on what is available for train services.