The direct couchette carriage only runs in summer. You can stil make the journey but with a few changes. See the timetable here:

https://www.bdz.bg/en/a/sofia-istanbul-sofia

(In the PDF at the bottom of the page.)

You can also go via Craiova and Vidin. You don’t need reservations on this route.

Hi guys, I am also trying to buy a ticket from Bucharest to Sofia (and also Sofia to Istanbul afterwards), both for me with a Eurail pass and my son who doesn’t have one. Despite what various sites say, there doesn’t seem to be anything available on the Romanian nor the Bulgarian national train websites. The link posted above seems to show an out of season train schedule, but they stop short of our travel dates (07-10.04). This suggests that the summer schedule is different, but I can’t seem to find that, nor work out how to get tickets. Any help much appreciated. Really want to try and get advance tickets (even if I don’t use my rail pass), rather than risk a full train or problems etc. as we are on a schedule and need to plan the accommodation etc all in advance. Thanks for any help!

 The timetable seems the same for your journey, see the pdf on this page: https://www.bdz.bg/en/a/sofia-bucuresti-sofia. It involes a change in Ruse.

The man in seat 61 seems to state that you cannot book this journey in advance, you willl need to buy it at the international ticket office in Sofia. Availability shouldn’t be a problem. Easter also falls later in Bulgaria (Orthodox, 16/04)

https://www.seat61.com/international-trains/trains-from-Sofia.htm

Good morning. As reported on the site https://www.bdz.bg/en/a/sofia-bucuresti-sofia there are no timetables for the route during the summer of 2024. I ask you if there will still be a direct train between the two cities or less so that we can regroup in advance.
Thank you.

There will be a direct couchette coach in summer:

https://www.seat61.com/trains-and-routes/bucharest-and-sofia-to-istanbul-by-train.htm