Likely Bosnia as still some working steam there, think near Tulza? No steam in Serbia (standard gauge), almost all non electric hauled freight in Serbia is in hands of class 661 GM locos.I'm sure I've seen relatively recent footage on Youtube of a steam loco on a container (or partly container) train in either Bosnia or Serbia. Frustratingly, though, I can't find it though there are films of steam on regular freight from as recently as 2019 or so.
Mine too...The short, timber-bodies containers on flat wagons (occasionally in low-sided opens) were pretty universal around the system by the end of the 1950s, before dieselisation. They could be seen laying lineside at many freight depots. Many individually in marshalled trains but there were several dedicated services. Goodness, my Hornby railway even had one
Not seen a 'Challenger' in the metal, but I have seen a 'Big Boy' close up - the (coal burning) firebox is enormous, as is almost everything else about it...The evaporation rate in the boiler must be huge.