Currently no Railtraffic from Greece to Serbia/Bulgaria/North Macedonia 

Montenegro is a dead end as the they have only the International Rail line to Serbia and no Rail connection to Bosnia or Croatia :/

Your options train to Thessaloniki then by bus to Sofia and from Sofia via Craiova or Ruse-Bucharest to Budapest/Vienna and then further to France for example from 12th December 3timer per week with the old new Nighttrain Vienna - Strasbourg - Paris :)

Thank you so much for your reply! Looks like I’ll need to go back to the drawing board! I’m travelling with my three children, we are starting in Thessaloniki, I don’t mind getting buses, was hoping to go via Macedonia, Montenegro then from Croatia through Italy. I’d mapped it all out on the rail planner but clearly that’s not going to work! Kerri

It work but then you have to use buses and catch from Bar,Dubrovnik,Split or Zadar a ferry to Italy or all the way via Slovenia to Italy