Budapest - Bucharest: either book via the ÖBB website or call ÖBB (+43 5 1717) and then collect your reservation in Austria (cannot be sent via e-mail).

Bucharest - Sofia: you can try via ÖBB by phone, or else book it when you arrive in Romania.

Sofia - Istanbul: only possible in Sofia.

Note that there is also a direct train Bucharest - Istanbul (unless you want to visit Sofia of course).

Super ! Merci pour votre réponse. Ça va beaucoup m’aider. Mais là, j’ai quitté l’Autriche aujourd’hui. Est ce que je peux réserver une fois sur les lieux ( Budapest - Bucarest - Sofia ) ? Si oui, est ce que mon Global pass sera pris en compte pour réserver afin de bénéficier d’une quelconque réduction, ou bien j’aurai à payer au prix normal comme tout autre voyageur lambda ? Pourriez vous donc m’expliquer comment la réservation se passe ,séance tenante, dans ces dits pays ? Merci d’avance. 

Where are you now? The reservation Budapest - Bucharest should also be possible at the ticket office in other countries, e.g. Germany, Czechia, Hungary, Switzerland, …

Just say you have an Interrail and want to book a reservation.

Ok great! I'm in Germany at the moment. Can the reservation from Bucharest to Sofia also be in Germany, as well as from Sofia to Istanbul? 

Maybe also Bucharest to Sofia. Sofia to Istanbul not. But you can always ask of course :)

Ok thank you, but Bucarest to Istanbul is possible ? 

Yes, timetable is here:

https://p.bdz.bg/www/s/o/sofia-istanbul-sofia-2-2911.pdf

But the reservation can only be made in Bucharest.

Okay, if the reservation can only be made in Bucharest, that means in Bucharest I can't pay in euros. So how do I do that? 

You pay with a credit or debit card (then always choose to pay in the local currency, never in euros. Your own bank will do the conversion). Or you first get cash from a machine. Or you exchange euros for Romanian lei.

Est ce que mon global pas interrail me permet de me rendre en Turquie ? 

Yes, you can travel in Turkey with your Interrail pass.

Cool thank you. And how about from Istanbul to kusadasi ? 

NO trains-there is afaik currently only a very limited Train service remaining in TR-it was never great and Kis-Ada-Si (=small island of si, a kind of bird) never had any trains. Read your Routard!

There used to be a rather slow+part ferry connection Ist-Izmir, but again-afaik it is still suspended. About only trains running reliably are the hi-speed (well, compare to TGV medium speed) Ist-Ankara/Konya.

Anyone overland in TR goes by BUS. Buy ticket at otogar. Do not have passdays remaining for local travel in TR as local tix are far, far cheaper.