All reports till now about this train tell its perhaps half full and very easy to get the needed REServ. Some people say they have even done it with the car-conductor.

Youre profile says youre BG=Bulgarian, so ask at any larger BDZ station that can handle INtern. ticketing.

Price wil depend on what you want-I believe its either seat or couchette, IF you have a pass (and do note that you can only use that for 2 days IN your own country-OUT and IN-and also that a pass is generally worthless for to use in TR-hardly any trains at all. Estimate: seat 4-5, couchette (a basic bunk to lie down) 14-15€ (double that in BGL)

You have to go the the INtern desk and show pass and tell what you want. THey have done that dozens of times and push some buttons and there is a ticket-then you pay.

Read seat61.com for more info also about Halkali and how to proceed-and if youre green as grass-how it all works in such trains.

Hi thank u so much for answering u really saved my life appreciated 

So does this train have a seat tickets? as i understand (4-5) means 4-5€ right.(excuse my english)

i have bulgarian citizenship but living and raised in istanbul(i cant speak bulgarian but as u said they've done this so many times so)Im just gonna use the in/unbound like normal days. I think (actually as i understand) if we want we can use these in/un things like normal days cause i have a interrail global mobile pass. 

As usual @mcadv is providing wrong information.
This train is having only couchette/sleeper. The reservation for couchette is 10 euro.
I arrived with the same train like you in Sofia and bought the reservation there without problem. You’ll have to show your Interrail ticket when buying the reservation. It was in May and train was almost empty. Now during summer it can be more full. On Friday and Sunday evenings it might be fully booked! But in that case you’ll still have the option to use the domestic seating carriage until Dimitrovgrad and take the Bucuresti-Istanbul couchette from there, which will be attached to the Sofia-Istanbul train in Dimitrovgrad. You should be able to get the reservation for this train also in Sofia...

@MartinM thank u so much

im gonna take the train on tuesday evening so probably it wont be full (i hope) if u dont mind can you explain the other option to arrive to the istanbul cause if a lose the train thag day i cant get back to turkey cause it has only 1 departure in a day Thank u so much

Other option is take the same train and change then in Dimitrovgrad to the Bucuresti-Istanbul carriage which is usually not very busy. If Sofia-Istanbul is full, just ask for a reservation Dimitrovgrad-Istanbul.