For domestic Romanian trains you should make the reservation in Romania I think. IR app is not able to sell this (and it would be way too expensive with the reservation fee compared to price in RO).

Bucuresti to Ruse depends on the conductor… sometimes they demand a reservation, sometimes not. I traveled this route a few times and ALWAYS got sold different tickets… once they sold me domestic reservation to Giurgiu (cheaper), once they sold me international reservation to Ruse (for normal ticket). In Bulgaria going back from Ruse to Bucuresti they sold me NO reservation (and ticket was also cheaper) and this year with Interrail from Ruse to Bucuresti also nobody asked for a reservation.

Thanks, but just checking - there was no problem in the Romanian trains accepting the pass on these routes and just charging for reservation, as I dont want to have to buy a full price seat...

also are ticket offices open all the hours in Romania as I will need at Brasov to buy a ticket at 7am. Thanks 

I didn’t say that there is a problem with using the pass.
I just explained that they always handle it different with reservations for the train to Bulgaria… 
No problem to get a reservation for this train on the day of departure. If there is no Ukrainian refugee wave, then this train is mostly empty.

In Brasov will be open for sure at this time. 
Not sure if 24h, but at 7am for sure.

Thanks, it seems Romania is not very compatible with the INterrail  website for reservations then! But I am making all these trips in working week monday-wedneday so I assume not busy ...

Yeah, shouldn’t be too big problem… In worst case you can go to conductor and get “unofficial reservation”.

Thanks MartinM. Out of interest have you also travelled on from Bulgaria to Istanbul, the overnight? Same situation it seems- I cant reserve that in advance, must do at Bulgaria’s Sofia Station?

You should be able to reserve it at any international ticket office in Bulgaria.
From other countries it is not possible AFAIK. In October shouldn’t be too busy anymore. Just Fridays and Sundays you should book ahead. Other dates should be possible to get something on the day. Reservation for 4 bed couchette costs around 10 euro (they also take cards usually).
When I used it in May this year (Sofia-Istanbul) I’ve made the reservation 1 hour before departure… But this summer I heard from other travellers that it was booked out days in advance (high season… and post-corona summer).

Note; IN RO the pass is ONLY valid on CFR=the age-old state owned national company, which mostly has quite old trains. There are also on the route Br-.Buc 2 or 3 other private operators, these do NOT accept passes and take only their own tickets (most are on the fone). They mostly offer much more modern trains with more amenities. CFR only sells tickets/RES for its own trains.

I'd expect DB to be able to book this (ticket office or by phone), using the train number search, just like they can book Bucharest-Istanbul this way.