In Novi Sad (9.40), you change to a bus to Novi Sad Ranžirna (9.50) included in your pass], from there (10.00) by train to Subotica (13.40), from there (14.02) by train to Kelebia (14.16), and you arrived in Hungary…

From Kelebia (14.40) by )rail replacement] bus to Szeged (15.35), from there (15.45) by train rseat reservation compulsory] to Budapest (18.07) .Budapest Nyugati pályaudvar]. It’s as easy as that…

PS: In direction of Vienna, it’s easier to change in Kőbánya-Kispest to Kelenföld and from there to Vienna (21.18) - just in case. 

Thanks a lot. It is still quite complicated and time consuming but at least I get there!

Note: the hourly Sz (nice place to stay in fact)-> Bp are part IC (with AC cars-must REServ-thats 300 HUF when done in HU) and part Fast=gyors (NO AC) and these cars are NON Reserved.

Note also that 2nd train in Serbije is said to often be cancelled-train broke. or its so late (zpozdeni) that it does not connect into 14.02 to HU.

In that case-go to real BUSstation-see why over there most people rely on that method-and pay for ticket. Many people are more sensible and book a direct bus all the way. Surprise is also that as for now Serb Rail does NOT accept passes on its brandnew shiny NSad trains, though promotional introduction fares are very low.

Thanks, mcadv, for useful info. Indeed, no pass accepted on new train Belgrade -Novi Sad (because of promotion, ticket costs only € 3 !). Good advice to take bus from NS to Kelebia (or even Szeged). Serbia still has a lot to learn about train travel!