almost all trains run all the year There are just few seasonal trains to touristic spots as direct trains (The Cities are still reachable in off season but with more changes )  :)

For example in summer a Nighttrain from Vienna to Split or Budapest to Rijeka/Koper :)

Only excpetion in my experience is the Train Belgrade - Skopje - Thessaloniki that operated since years just in summer (Actual still not back in service due the pandemic )
 

There are a number of seasonal trains. The best is to verify a timetable on the operators website, not in the Interrail app.

If you have a question about a specific train, then please ask a specific question.