Train travel in the region is difficult at best. Not sure what your goal or interests are or your budget for that matter. If you have the ability to hire guides you can cover a lot more ground in much less time. 30 days is great, but not if you spend 15 days of the trip on trains creeping along at 40mph, stopping ever 10km to pick up more passengers. A guide with a car in most of the countries you mention might run $200 a day; or less.
For Hungary because all train tracks go through Budapest, if you want to see anything other than Budapest you have to either keep bouncing back to Budapest, look at bus routes or again hire private transportation between the various towns. There are a lot of great places out ide of Budapest so give it a hard look. For Romania you take a commuter flight from Budapest to Cluj and start your ground trip there; on our trips we flew to Bucharest and then drove back to Budapest. Great drive.
We've been to Bulgaria a few times and would return again. Bulgaria is a great door to Istanbul.
I've done a lot of research on Serbia and just haven't gotten excited enough to give it a try. Montenegro has some beautiful areas and the fly fishing is great. Parts of Albania and F.Y.R.O.M. are on our radar and I've done a lot of research but we haven't bitten the bullet yet.
The route? All depends on your interests. I can recommend a few decent guides if you are interested.
