There is no general answer :)

Usually the staff at the ticket office know how to handle Eurail and book then the reservations for you. (But there are still few that are not well trained and are not 100% how to handle reservations. If the ticket office use the International System you could ask them to use the Tarifcode 44 it´s the tarifcode for Railpass user)

Another reason why they try to sell full fares is that the Reservations for Eurail/Interrail user are solt out. Especially International TGV´s , Thalys & Eurostar have just limited seats for Railpass user and are often sold out. Happens actually quite often for Eurostar services from/to London and the TGV´s from Paris to Barcelona and Paris to Milan :/

If you let us know which Routes you have in mind we could say which reservation fee´s you can expect :)

Thank you for all that info! The first one im looking at is the night train from Budapest to Brasov. Also Bucharest to Sofia, and Budapest to Krakow. Thanks!! 

Thank you so much for all the links! One last question, are these prices the normal train prices or discounted for interail passes only?

The prices you see are the reservation fee´s for Interrail & Eurail :)

How ever railtravel in east europe is quite cheap :)  Standard fare Bucharest - Sofia is 25€ :)

Great, thanks so much again:)

Note: as for now-and for some time to come, NO overnight and even NO direct train Buc-Sof, change Ruse (=first stop in BG). In fact that 25€ can even be further reduced by just buying INT ticket to Ruse and local BDZ ticket onward to SOF.

In general in these areas=East EUR there are no real issues with using a pass or booking the REServations-except that it may in the end be cheaper to buy local tickets as using passdays!