It’s too early. Seasonal summer trains show up much later and the rail planner app is to be avoided. Best use local planners or a planner from Deutsche Bahn or ÖBB. But in General, if you check  your trains about 3 months in advance, you get to see the correct itinerary.

If not, a lot of people in the community can guide you to the cross-border busses in case they shouldn’t show up.

Here’s the timetable for the international train from Sofia to Istanbul:

https://p.bdz.bg/www/r/a/razpisanie-sofiya-istanbul-3580.pdf

Should be the same in summer. Kind regards, Hektor.

Thanks Hektor for the quick reply. Any idea on when they might be available to book? 

Probably 60 days in advance. But only in Bulgaria, as far as I know.

Thanks again, great community  

As this scheme also shows only in summer-late june till half sept roughly, this train will also convey a very few through coaches from Bucuresti-one of the wonders of trains: they can couple-and uncouple cars.

FROM BG you can only RES-and its couchette=a type of dorm-sleeper, in Sofia, this office was even reported to be closed on SUN. Any spare space can also arrange via car-conductor. At least no worries for now about trying to do bookings that cannot be done. This train was always said to hav enough space-even on the day-but last summer in the then travel-boom was sometimes completely full