Train from Sofia to Belgrade?

Thank you that's very helpful.   I have planned a big bus journey passage to complete my route as otherwise I would have to miss out a lot of cool places.  

Any idea on when trains may be back up again?  Meant to be travelling through the area around July 22nd

Nobody knows… But you can check the websites of Bulgarian and Serbian railways.

I think, there are works on the railway line, because on the serbian part of the railway line (Niš - Dimitovgrad) there are no domestic at all. But Sofia - Dragoman is operated, as well as Niš - Belgrade.

But maybe ask directly the serbian / or bulgarian railways

Nis-BG is now (or soon to come) also under big repair/renovation, I just read yesterday-after they have finished the now under ditto const line toward B-pest. In fact in Serbije nearly the whole remaining rail-network is out of use or under reco.

Hint: it makes no sense to only cover the gap between railheads-take a direct bus instead, much, much more quick and easy and comfortable.

There are dozens of combined sites that offer to show all buses running between A to B-esp in Balkans-but even these do not show all existing connections and no, you do not need to make such bookings monthes, weeks advance.