Interrailing around Eastern Europe - Locations?

Almost none suggests the Balkan flexipass because of limitations in train routes and better coverage by bus service as well as trains being relatively inexpensive.

It would be easy to spend your entire four weeks in Turkey. In fact, I would encourage you to consider this as your other destinations are so widespread and deserve time to see them as well as some of the places you will pass along the way. You mentioned 9 destinations in 30 days. What with travel time, that isn't much time to see things.

You might want to reconsider these plans.

Ruth

This is a lot of ground to cover in 4 weeks and I think you would end up seeing most of it through the glass of train or bus windows.So I agree,you need to cut out some places.

I'm not familiar with the other countries but I can tell you that once into Turkey you can take the train from Istanbul to Izmir and change there for another one to Selçuk Ephesus.But...(nothing counts before the but!)from Istanbul it is ferry/train combo via Bandirma and there is a long wait between the ferry arriving in Bandırma and the train leaving,c.6 hours.So you would spend a whole day to do this.A pleasant enough journey but frankly you could use the time better.Either by day or night bus(9 hours) or just fly down in 1 hour.
Flights are plentiful,frequent and very cheap,often the same or little more than the bus or train.
There again maybe you just enjoy train travel in which case you may be prepared to wait around in Bandırma :)
I have never used a Balkan rail pass so don't know if it would be worth it or not but trains here inTurkey are very cheap anyway.

In Turkey the rail system is a bit slow sometimes but does have some wonderfully scenic routes even going right over into eastern Turkey.Sleeper trains are clean and modern with good meal services on board.Slowly but surely many routes are now on high speed rail systems too.
There is some good info on this web site-
http://www.seat61.com/turkey

Novi Sad is certainly well worth a visit in Serbia, its the Capitol city of Vojvodina, a lot of history, a large fortress over looking the Danube & Novi Sad from across the river, excellent museums, nice city centre, not as busy as Belgrade, more of a laid back atmosphere to the place, its on the rail line from Budapest to Belgrade, so easy to get to, get the right train and theres a nice dining car on board, quite cheap but okay quality wise. Plenty of good bars and cafes.

If visiting Belgrade then you must also go Zemun a small but much older city which is now surrounded by New Belgrade, but is very easy to get to by Bus from the centre of Old Belgrade. Zemun is on the shore of the Danube, loads of cafes and resturants, lots of old pretty buildings and things to see, its where people from Belgrade go for a day trip, but only a short bus ride, buses run every few minutes.