1. Online booking is not possible. Ask at a ticket office where they sell international reservations, maybe even SNCF can do it. If you're passing through Germany or Austria, call DB or ÖBB (+43 5 1717) and collect the reservation in Germany or Austria.
2. Again, booking a reservation online is not possible, only regular tickets can be booked online. Since there is no pass quota, that means you can see availability of the CFR website. Buying a reservation is only possible in Romania. Currently there are still places available.
   If they can't sell a reservation anymore at the ticket office, then ask the couchette coach attendant (you'll probably have to pay cash). If that doesn't work, then take the same train on a seat to Dimitrovgrad (you'll have to change in Gorna Orjahovica). In Dimitrovgrad, the carriages from Sofia are attached, so you can again ask the their attendants if there are any free places. Only couchettes and sleepers continue to Halkali, so you'll need to get a place there.

For Turkey, the best is to book as soon as possible a normal ticket online and exchange it for a pass reservation once you get to Turkey. See this post:

Since you want to travel to Halkali on a Sunday night, and that can be a busy night, please have a plan B in case you can't get a couchette or sleeper to Halkali.

In addition Geneva ticket counters (CFF/SBB) should be able to sell Budapest - Bucharest. Feel free to ask if the queue is not too long…

No other options than given above for Bucharest - Halkali but you should get a couchette somewhere.

Thank you both for your help. I will try the Swiss rail counters on the day of my departure, I can always leave an hour early to make sure I have time.

Thank you also for the correct link to the Turkish rail ticket purchase, I had found myself on a different site for the same company that didn’t recognise anything East of Ankara as a valid station...

Fingers crossed that this Romanian section isn’t too much of a hassle, I feel like getting stuck in Dimitrovgrad would really not be ideal.

Last question : When you suggest asking the attendants directly on the train and paying cash, do you have a rough estimate of how much this will cost ? I would need to make sure I have enough Romanian currency….

The couchette Bucharest-Istanbul is operated by Turkish railways, so they might also take euros. I'd be at the platform early to ask the attendant (check in advance where there's an ATM in the station so you get get Romanian cash quickly if needed). Price should be around €14 or RON 70.35 according to the CFR website.

Thank you very much for your help.

Please note that the direct line between Budapest and Bucharest is currently closed due to an accident:

https://www.mavcsoport.hu/mavinform/bekescsabai-vonalon-hetfon-korlatozasok-kesesek-potlobuszok

Some international trains are diverted, which takes time, so plan enough buffer.

Ah Very unfortunate. I suppose there’s a chance it will be solved by then, I imagine I can stay updated on the mavsoport.hu website. Thanks !

Update from Romania :

I would not recommend this route to anyone at the moment. The rail accident is still not solved, there are now two options for Budapest-Bucharest :

Either take 2 trains and a replacement bus to Mezotur, without a booking for a couchette (due to the incident the ticket office in Budapest cannot book you a place !! They are also not informed about the situation and delays etc).

Or taking the 22:45 which runs directly from Budapest but takes a big detour through Cluj-Napoca. Nobody can tell you in advance how much delay this will cause, but so far it is looking like we I'll arrive around 20:30 instead of 15:10.

It also looks like the Bucharest-Istanbul will be full the next 2 days which means I will be stuck there, but I don't see how I could have proceeded otherwise...

Ah not really good with the accident… :/

Between Bucharest and Istanbul my advice would be to follow the couchette carriage throughout the day but instead on regular seating carriages. Keep in mind that you'll have to change trains but you'll have enough time for that while the couchette carriage is shunted around. Then in Dimitrovgrad it is coupled to the Sofia - Istanbul train which is much longer and should have couchettes or sleepers available.

First ask at Bucharest train station of course.

Although the couchette carriage may seem full on the CFR website, always ask the couchette attendant.

They finally managed to solve the situation in Hungary, the line is back to service. If you look at the pictures, it was a really complicated derailment…

Now for the Bucharest - Istanbul trip…

Even in a normal situation (with plenty of couchette places available), I recommend the bus on this route, and more specifically the Lilian Express service. I used it three times in recent months. It is much faster than the train (11 hours, instead of almost 20 hours for the train), very comfortable, with reclining seats, and a steward serving complimentary water, tea, coffee, and snacks. Contrary to other companies, they do not carry goods, only the passengers personal luggage, so they pass quickly through customs. And in Istanbul they arrive at the main bus station, which is much closer to the center than the suburban Halkali station, where the train arrives. The price from Bucharest to Istanbul is around 40 euros. They depart in front of the Filaret bus station, at 19 hours every day.

I will ask at the station and try my luck as well tomorrow morning.

How easy is it to figure out "following the sleeper carriage" around until Dimitrovgrad ? Bearing in mind I don't speak Romanian (or Bulgarian) and so far have met very few english-speakers... 

I will ask about the Sofia-Istanbul at the station, do you know why it doesn't appear in the interrail planner ?

I'm going to book my way back as well so I don't have this problem coming back. There is 1h30 changeover time between the Istanbul-Bucharest and the Bucharest- Budapest... That seems a little tight to me, would you recommend planning 1 day buffer in between?

Thanks for all your help

Have a look at this page for schedules and general info : https://www.seat61.com/trains-and-routes/bucharest-and-sofia-to-istanbul-by-train.htm

There should be multiple night trains between Bucharest and Budapest if all trains now run again. I'd plan for more than 1h30 if possible.

You'll have to book Istanbul - Bucharest in Istanbul

Thanks for the link, I understand the suggested change much better.

No luck at the counter, it's fully booked the next 3 days ! I can still try to he attendant directly but I don't have much hope.

They're also completely unable to tell me if the Sofia-Istanbul is fully booked, is there any way I can find out ? I've tried the Bulgarian website but it doesn't want to accept Istanbul as an option maybe it's just for domestic travel.…

If I can't be sure I won't be stuck in Dimitrovgrad for 3 days I will probably just pay for a bus to Istanbul…

Thanks again, last questions I think !

AFAIK reservations from Bulgaria to Halkali/Istanbul can only be done at a Bulgarian ticket office.

You'll have a chance in Ruse, first station in Bulgaria, where your train will be standing from 13:45 to 14:20. They have an international ticket office.

Hi everyone,

Reading this thread as my final destination will be Istanbul, is my understanding correct that to hop on a train heading Istanbul the reservation should be done in person at any ticket office that sells the istanbul tickets ?

Is there an easy route to Istanbul from anywhere in europe (or starting from Vienna) where reservation can be done online? 

Thank you for clarifying this for me in advance, as reading the thread above make me want to just fly to Istanbul and ditch the Interrail pass altogether. 

The Vienna - Bucharest night train leaving at 19:42 can be booked on tickets.oebb.at : add Interrail as a discount. It stops in Budapest at 22:42 too. You should be able to book all other night trains from Budapest at a German or Austrian ticket counter. Maybe in Switzerland too.

The last journey from Bucharest cannot be booked online yes. Either you wait until you arrive in Bucharest and worst case you sit on seating carriages throughout the day until Dimitrovgrad or you buy 2 regular tickets for Bucharest - Istanbul on CFR's website. This is an easy solution if you're worried.

No need to fly :)

Update from Georgia (beyond Interrail but final destination) :

I ended up taking a bus from Bucharest to Istanbul, there is a Flixbus every night. It's an extra cost on top of the pass but less than buying a full price CFR ticket. (if you book on CFR you will have to physically pick up your ticket at a train station). The bus also takes less time (billed as 10 hours but took 12h, border crossing times are variable).

I did go to see the 10.50 train in Bucharest just in case, no wonder it fills up for days ! The couchette wagon (singular) is tiny and half the space is taken up by seats and tables for the non-sleeping time. Looks like travel in style for the passengers but not very space-efficient.

I was able to book Istanbul-Bucharest as well as Bucharest-Budapest for my return directly in Budapest.

Turkey : do indeed book tickets online or at least check availability very regularly, they are highly serious about 1 booking = 1 seat even in second class seating for short trips.

However unless you speak Turkish I don't think it's realistic of them to recommend getting someone at a ticket office to exchange your full price ticket for an interrail booking. Tried 5 ticket booths, no luck, half of them didn't even seem to know what Interrail is. Not such a big deal given how cheap the tickets are. 

I really recommend the Ankara-Erzurum(-Kars) train through eastern turkey, best experience of the whole trip, it goes through the most beautiful places.

That's not how it works. All compartments have a day-time and a night-time configuration.

Did you ask the couchette attendant?

Thanks for the update! 
Yeah, even being with a Turkish speaker like my gf it isn’t easy at all to exchange it… but when they finally understand it, then it is done within like 1 minute. 

And yeah, the Ankara-Erzurum-Kars train is indeed wonderful. Same goes for the Ankara-Tatvan train, but that area (nearby) is maybe a bit less safe than Erzurum and frequent police checks if you travel further (south)east. On a bus ride from Van to Erzurum they checked my passport 7 times!