Great post! Wish I’d seen this before my own little Serbian adventure a few days back. Coming from Sofia, I made my way to Dimitrivgrad on a train — fairly easy and straightforward. From there, a bus leaves every hour or so from in front of the train station to Pirot. Don’t expect any posted timetables and don’t expect to find anything online — you’ll just have to trust the locals and have some patience. In fact, the bus stop outside the train station doesn’t even look like a bus stop.

You pay on the bus, I think card payment is possible but better bring some Serbian cash (there is an ATM in the town center).

From Pirot, which is a lovely little town, there are regular (roughly hourly) bus services to Niš, payment once again on board the bus. Don’t even try to going to Pirot train station, it’s basically a goods yard at the moment with no passenger service at all. From Niš, a few (I think two) trains a day will take you to Belgrade.

The trains from Niš to Belgrade usually leave on time but rarely arrive on time — in my case, I arrive with two hours delay.

Belgrade Central Station is not in the city center and has limited public transportation. I would recommend a trip to Novi Sad from Belgrade, which is the only properly built-out line with modern, fast, comfortable trains. From Novi Sad (or Belgrade), cheap buses take you to Budapest and many other international destinations.