Do you have any idea where/how to book a ticket?
About ticket purchase for the Russian carriages MAV on their website says: "Hálókocsi-foglalások mind odaútra, mind visszaútra kiadhatók az
EPA-rendszerből!", however recently it was reported that there
had been some troublesMaybe also special codes for issuing such tickets will help (took it from MAV itself)
Is it possible to buy a ticket from the provodnik on board? Many years ago I travelled from Budapest to Venice in the RZD sleeper paying the provodnik on board. I did have a ticket though because that time it was not a global fare yet
Hmm...good question. Trouble with Russians is that even if it might not be possible de-jure, it can still be possible de-facto (=paying provodnik "unofficially"). Just searched for the docs regulating it, but no success. Anyhow, if we would try to guess, chances of buying it officially are slim