There are no seats to Istanbul, only couchettes and sleepers. There is a seated carriage as far as Svilengrad.

International ticket offices in Bulgaria are listed here:

https://www.bdz.bg/en/a/stations#tid-11206

Karlovo and Dimitrovgrad are not in that list. Plovdiv is. You can try in Karlovo but don't have much hopes.

Reservations will however close a certain time before the train leaves Sofia. I don't know how long in advance.

This service can be busy in summer, especially at short notice (1-2 days in advance) and for Friday/Sunday departures. Any free beds/berths can always be booked on the train.

thank you! I stop in Ruse for about 20 minutes on the way to Karlovo (from Bucharest) - do you think I could get out of the train, go to ticket office to make reservation, then come back to train?

Ruse has an international ticket office, so by all means, give that a go.

The stop in Ruse should be from 13:39 until 14:15.

Can I pay with card at the ticket stations?

do they generally speak English and understand the interrail reservation stuff?

Nowadays you can pay with card at the stations.

In international ticket offices they usually speak some basic English. In domestic offices mostly not.

Update: I spoke to a rail operator, in early August when I’m going there are rail upgrades so the sleeper does pass through Plovdiv, but actually through Karlovo (though no time is given on the new timetable so maybe passes straight through) and Tulovo… BUT the guy said that there are *no seat carriages*, even to Dimitrovgrad, which I’d heard was not the case? So I suggested I could go from Karlovo to Dimitrovgrad by seat and then by couchette but sounds like I can’t do this, so would need to take a seperate domestic train from Karlovo to Dimitrovgrad via Plovdiv… which is having rail upgrades!!