Sure that's no problem. Reservation for day trains Budapest  Vienna is optional by the way.

Besides the hourly OeBB/MAV combo trains (most are very nice RailJet) there is also 2-3 x/day a yellow private train of RegioJet-going on do Prahy. -this does not take passes on this sector, but normal fares are quite cheap-if this is the only trip you make that day it may work out as better buy.

As rian tells: the hourly trains CAN be but do not NEED to be REServed-though it is mandatory for trips IN HU only-strange enough. You can use a yellow MAV machine and RES till last stop-for 330 HUF or so (less as 1€)-then you are also seated.

Alright, very clear !

Thank you for your responses :)

@mcadv Regiojet accept Interrail Passes on their trains, except for the train Prague-Bratislava-Split/Rijeka. 

RegioJet lines where Eurail & Interrail can be used:

• connestions in category RJ:
  • RJ Prague-Brno-Vienna-Budapest
  • RJ Prague-Brno-Bratislava
  • RJ Prague-Brno-Žilina
  • RJ Prague-Brno-Košice
  • RJ Prague-Ostrava
  • RJ Prague-Brno
• connetions in category R: express lines R8 Brno–Bohumín and R23 Ústí nad Labem - Kolín
• Prague-Krakow train/bus (reservation required)

Prices for booking a seat:

• Low cost tariff = 1,3 EUR
• Standard tariff = 2,0 EUR
• Relax tariff = 2,8 EUR
• Business tariff = 1,3 EUR (can only be ordered by the 1st class holder)
• Bus seat = 1,5 EUR
• Stated prices apply even if the passenger uses only a section of the entire line's journey.
• Book a seat at any RegioJet or STUDENT AGENCY sales point or online.

​@rvdborgt  I tried making a reservation for 2 seats / beds on this nightjet (Budapest » Vienna) but independingly of the chosen date not a single seat is available. If I remove the Interrail Pass, all kind of seats are available for for countless dates, my travel date in April, next week, in July, no matter what I choose.  Do you have any idea? Thanks a lot.

Budapest-Vienna is too short for a night train; it's less than 3 hours, so I that's why they don't sell sleepers and couchettes. You can use the coaches with seats. These run as EC train, have optional reservation and can be booked for €3 if you wish.

@rvdborgt : Ah yes of course… But actually I ment Augsburg, I’ll travel with my daughter from Jena to Vienna to Budapest and back home to Jena via Augsburg. Sorry, I was a little confused.   And this was not available. But I found a good tipp here in the community: by choosing "one way and day tickets" insteadt of "seat reservation only" I was able to select the seats. Bye