The Eurail map is a lie in this part of the world. I was livid when I found out. There is only one international train running from Serbia and that is to Barr, Montenegro.
The following trains have been cancelled and you must buy a (long) bus connection:
Zagreb - Belgrade
Budapest - Belgrade
Belgrade - Sofia
Belgrade - Pristina
Almost every train in Bosnia. The only train that runs is Mostar to Sarajevo, NOTHING ELSE.
Many domestic trains in Serbia. Eurail is not accepted Novi Sad - Belgrade. You can get to many borders on domestic trains, but schedules on each side do not work out. You will have to find your own way across the border and overnight in the border city to catch an onward train in all sides other than to Bar.