The itinerary lasts 1day 17hrs 50min, and has 3 steps, and it will cost 65 euros minimum.

Brussels - Vienna
The step from Brussels to Vienna lasts 14hrs 00min. You'll travel in a Nightjet operated by ÖBB.

Vienna - Bucharest
The step from Vienna to Bucharest lasts 18hrs 20min. You'll travel in a night train.
You can find tickets from 65 euros.

Bucharest - Sofia
The step from Bucharest to Sofia lasts 9hrs 30min. You'll travel in a regional train.