REgionaltrains doesnt need a reservations
Fast Trains like Intercity or Nighttrains need a reservation
Reservations are from 1€ for domestic journey on daytime via 3€ International daytime reservation up to 40€ for a sleeper reservation.

Serbia & Montenegro is difficult to reserve in advance Many years Mr. Popvic was the only option but i heard nothing from him since few months so iam not sure if he is still active :/

I believe I recently read about him again on DSO.

There are hardly ANY trains INside these 2 countries-most are cancelled. There is 1 connection between the 2 left-overnight and only summertime by day-said to be a very scenic route-this is about the only train you really MUST reserve-and in hi-summer it seems to be in demand. It is also the only 1 train going even over border to these 2 countries. So using a pass there hardly makes sense.

Also note the old central station in Bgrad is no more-train go from a bleak suburban stop without even a decent bus/.tram going there.

Somehow I recall that this mr (who had a kind of business doing special train requests for foreignrs) is now in pension but if its worth it he still can organise something-you make a date when to meet him+pay etc.