There are currently no trains from Croatia to Serbia and no international trains to Greece. Nobody knows when that will change, although I'm afraid it will take a few years for any train to return for the route from Belgrade to Thessaloniki, because of the engineering works going on. As to Zagreb - Belgrade: I don't know why it still doesn't run.

Nevertheless, you could ask Slovenian, Serbian, North-Macedonian and and Greek railways about the plans. If you get any replies, do post them here.

The Zagreb-Beograd line was once the major axis of Yugoslav Railways=JZ. IN Hrvatska=Croatia the line still functions till what is now the last town before border; Vinkovci. But Serbije Railways have stopped-partly due to covid nearly all longer distance trains, only the short banlieux line around Beograd functions. The line toward HUngary is under heavy reconstruction by the Chinese.

I highly doubt you will get any response from these railways, like rian=above the regular people giving answers mostly rely on own experience and news in specialist forums. Also check seat61.com-it describes in /en/ all overland routings from UK to all EUR countries.

The only feasible way to travel over there by now is by bus. A site like balkanviator and several others have more details-but not too long advance-it can change at very short notice!

Small comfort: quite often a normal busticket over there is even cheaper as 1 travelday from InterRail.

Thank you for your answer, 

And, don’t you know if it’s possible to reach Greece by another way ? 

I’ll contact them, thank you

For the part Zagreb - Belgrade, you can do the following route: 

Take a train from Zagreb to Vinkovci (for example at 7:50 - 12:27)

From Vinkovci, take a local train to the small Bordertown, Tovarnik (13:52 - 14:22)

Cross the Border by foot or taxi

Finally take a local train from Šid (Serbian Border City) to Zemun near Belgrade (19:12 - 21:06)

It is also possible that in the next months the direct train Ljubljana - Zagreb - Vinkovci - Belgrade retourns (https://rs.n1info.com/biznis/na-prolece-ponovo-vozom-od-beograda-do-zagreba-i-ljubljane/). But nobody knows an exactly date :/

For the route serbia - greece, i can't suggest the train (except you want to cross the border twice by foot / taxi)

In the balkans, there are many good bus lines, use a bus for this route