The itinerary lasts 23hrs 40min, and has 3 steps.

Paris - Zurich
The step from Paris to Zurich lasts 4hrs 10min. You'll travel in a high speed train.

Zurich - Zagreb
From Zurich, take the 8.40pm train to wake up refreshed in Croatia the next morning, arriving in Zagreb at 10.39am.
The step from Zurich to Zagreb lasts 14hrs 00min. You'll travel in a Nightjet operated by ÖBB.

Zagreb - Belgrade
The step from Zagreb to Belgrade lasts 5hrs 30min. You'll travel in a day bus.