The journey between Belgrade and Istanbul is made on-board the Balkan Express. The service offers both standard first and second class seats, but also first and second class sleeping berths. The service is operated by Bulgarian Railways, using relatively old, but comfortable trains.
Part of the journey is shared with the Bosphor Express, operating between Bucharest and Istanbul.
There is an option to continue your journey on-board the same train to/from Budapest, Bratislava, and Vienna.
For further information, try this page at Turkey Travel Planner.

