There are no international trains to and from Greece at the moment. The easiest way which includes a night train is as follows :

• Thessaloniki - Sofia bus (Flixbus but there may be other companies too)
• night in Sofia
• Sofia - Bucharest 07:05 - 17:08 (you may or may not need to change at Gorna Orjahovica but the connection is guaranteed and timings remain the same). 1.25€ seat reservation (unlimited availability at Sofia train station)
• Bucharest - Budapest 2 night trains : either 18:25 - 08:50 or 21:46 - 12:50

For those night trains here are your options to book it :

• Bucharest railway station
• Interrail reservation service (but 10€ extra fee as seat reservations need to be mailed to you)
• maybe calling DB and having the paper reservations mailed to you (still cheaper than via interrail.eu)
• some option I’ve missed

Or you spend a night in Bucharest and then take the 13:50 - 05:20 night train which is available online through tickets.oebb.at (no booking fee)

Other route : ferry to Italy from Patras or Igoumenitsa and then ferry from Italy to Split + night train to Budapest (doesn’t run daily though)

thank you very much. On the interrail app, the only route from Sofia that is available is through Craiova in Roumania. Do you know this route?

What is DB?

Please do not use the Rail Planner app to plan, it is very unreliable and doesn’t show all connections. bahn.de (German Railways website) is connected to the European-wide timetable and is way more reliable. The connection via Craiova needs a train change in the middle of the night. What would be your approximate travel date so I can check ?

DB means Deutsche Bahn (German Railways) :)

ok thank you. Maybe do you know if there is a route avalaible through Skopje-Beograd-Budapest?

No international trains at all on this route unfortunately. So you'd rely on buses

If you don't want to go via Sofia or use buses, then there is the ferry to Italy option