Upvote:4
I entered your found train into the German travel site, and found that same train. Then I clicked the โ arrows, (the ones that reverse the direction of search) and the results were alike. One day train and one overnight. 11:05 to 17:32 and 23:48 to 5:52.
I would suggest you to run the site for a day you want to travel, I checked it for a couple of days from now.
Internet search only but I do find the German rail site rather reliable. (I would try to book through one of the involved national rail sites if possible, not the German one.)
Upvote:6
You can find the schedules via the website of the Serbian Railways. Note that you have to use the Serbian spelling of Belgrade, i.e. BEOGRAD: