National Rail's 'official' partner company, for booking European rail travel is Rail Europe. I'll give you the web link but it's a lot easier to phone them:
http://www.raileurope.co.uk/
However, the real experts when it comes to European rail travel are Deutsche Bahn. Their website lets you plan rail journeys throughout Europe:
http://reiseauskunft.bahn.de/bin/query.exe/en
It's possible to book online for some journeys found on that website but you'd probably have to contact Deutsche Bahn by phone to see if it's possible to book tickets for jouneys in Eastern Europe. They've got a booking centre here in the UK. (Unlike Rail Europe, they don't charge a booking fee for their services). Contact details are here:
http://www.deutsche-bahn.co.uk/db_uk/view/inde x.shtml
Hopefully, Deutsche Bahn will be able to help you. If not, these links might be useful to you:
Hungarian Railways:
http://www.mav.hu/
Timetable for Hungarian Railways in English:
http://www.elvira.hu/elvira.dll?language=2
Serbian Railways:
http://www.serbianrailways.com/
Croatian Railways:
http://www.hznet.hr/ENG/index.htm
Chris