https://www.seat61.com/trains-and-routes/belgrade-to-sofia-by-train.htm
 

 I’m in Sofia right now, the info by Seat61 is still correct. I also checked distance between bus and railway station. Takes 40 minutes walking. Does not leave much time to catch the 12.00 o’clock bus to Nis, if not fully booked anyway. So it might be necessary to sleep at Dimitrovgrad. Tomorrow I’ll check out the buses again. Bus might be best option, or to Nis or to Beograd. 

Dont mess with these ´it has to be train amap´-just find a bus all the way-not always online. Why that is messy escapes me-the savings are also minimal or you may even pay more as a direct bus if you need to use taxi etc. Plus that Serbije railways is also famed for very last min. cancellations-train broke and the like. Also DO be aware of the new ´station´ in Beograd-the old one besides the BAS (still there for the many more buses) is out of use. There is not even a decent citybus there to link that new station with centre town. Train to Bar for now in winter limited to only that overnight.

Thee are no trains into Serbia from Sofia. If you are in Istanbul ensure you book the sleeper to Sofia. This train can be full especially at weekends.

There are, but only to Dimitrovgrad:

https://www.bdz.bg/en/a/sofia-beograd-sofia

Which is the border. No trains beyond! I should have said no trains to Belgrade!

No serbian trains to Bulgaria. There is a bulgarian train to the city Dimitrovgrad, which is in Serbia. Why there aren´t serbian trains from serbian city Dimitrovgrad to Belgrade?

Engineering works between Dimitrovgrad and Niš. I don't know how much time they still need.

Because Serbian Rail upgrade their entire Railnetwork Belgrade - NoviSad took almost 4years and i assume that the Works on the Route Belgrade - Nis - Dimitrovograd + Nis - North Macedonia take even longer. Currently most trains between Belgrade - Nis use a different Route as usually.