How long dies it take to travel from Belgrade, Serbia to Bucharest, Romania by train?

Looking for timetables in advance and it says 24 hrs on average but that the train goes to Budapest for another train that leaves from Budapest to Belgrade. Isn't there a train that goes from Belgrade to Bucharest like it says in the Romania trains info on the eurail website and doesn't it take an average of 12 hrs if infact the eurail timetable map/picture is right?
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