I went from Belgrade to Aachen about 15-20 years ago. It cost me more than my plane ticket from Dusseldorf to Belgrade (though I got robbed/scammed at Belgrade). From Belgrade to Croatia the old DB train went 60 km/hour. It did have power sockets. I was nearly alone. Then came the evening in Croatia, the train went quicker than 60 hm/hour, people boarded (youth with backpacks), and sleep got a bit more tough. Near Ljubljana and Wien there were very bright lights at the station, and more people boarded till the 6 seater was overburdened with people trying to sleep yet take care of their belongings. The whole trip took near 24 hours. But if I was with a partner, it would have felt a whole lot more safe, so that is my recommendation: go with a friend and make sure you set checks & boundaries beforehand.
Don't remember, I put one laptop bag around ny middle, and larger backpack I put my legs into. However, it can be removed by a knife and opened if enough are in on it plus those not sleep. With a friend you can take guard turns.
