(train times are added at the bottom for those who don't want to read)
Serbia is not impossible, it is awkward but if you are determined to go it's fairly cheap and easy once you are in.
My experience
So the trains are a mess here. Nothing is traveling internationally except the BAR train in Montenegro. Most likely you are coming from Sofia, Budapest or Sarajevo. You will want to get a bus into Novi sad, Belgrade or Nis from there the trains rún semi frequently but delays are common and you will crawl through the countryside. The Belgrade to Nis train travels at 30km per hour. The Novi sad line is not much better.
No ticket vendor will help you, just don't bother asking anyone behind a desk. The locals are more friendly though. Buy a coffee and ask your questions there. Luckily the information is pretty easy to find and is reliable. If you leave yourself plenty of time you should be fine.
Now i knew things were a mess before I left Budapest for Belgrade and since this info was not online i wanted to take photos of the timetables so others can plan around it. However planning is not easy for Serbia giving the delays. My advice would be if you are experienced at traveling and know how to change your plans then Serbia is doable (not sure if it's worth it) . If you need to have everything pre planned leave Serbia untill they fix the lines. The up side of this is no one is traveling in Serbia right now. Busses were half full and i only met two other traveling groups. So your hostels are cheap and empty.
The timetables
So yellow means departure and white is arrival (taken from Belgrade central station)
Nis station
Hope this helps you, safe travels.