My Informations from Germany is that the train is currently not in Operation Maybe aske North Macedonian Rail directly :)
As @seewulf said in Balkan countries, its best to ask railway companies directly, before each journey. The information on the website can be correct, but dont have to be. In this case it is mztransportad@mztransportad.com.mk
they answer quite fast.
If you are in Skopje, you can also just go to the train station and ask. Perhaps you visit the Matka Canyon, the busses also start from the trainstation.
My Informations from Germany is that the train is currently not in Operation Maybe aske North Macedonian Rail directly :)
On the Macedonian railways’ website, the line for Belgrado is empty in the departure and arrival tables:
\https://mzt.mk/?page_id=898&lang=mk