I've taken the other half of the train once (the train from IST splits into two at the border, then half goes to Greece, the other to Bulgaria). It was a very nice overnight train. Of the 6-8 cars, 3 (1 Turkish, 2 Bulgarian) ended up going to Sofia. The Bulgarian cars did not go any further, while the Turkish car got another two or three added on for a trip to Belgrade and then on to Vienna. I'm sure the train composition changed along the way again somewhere.