To be more exact: in fact as for now the only train-replace connection Srb-HU is by BUS-till Szeged in HU-the whole route is under reconstruction by the Chinese.
In short: forget a pass in this area-the bus is the main form of transp for those without even a 2nd h car and the very few trains that should run in theory are often cancelled-train broke, no parts. And IF you find one running-the normal ticket is much cheaper as a passday