>>132377
Yeah, the high-floor railway rolling stock is a big theme for arguments among Russian railfans. The main reasons for it is that we have too much high platforms that the low-floor rolling stock would be fairly underutilized and hybrid rolling stock with high and low doors would have too much issues in operation so it is much easier to equip it with elevators (pic related, I mean). It's not that simple actually, there is tons of reasons for and against, I'm not gonna adduce them all.
As for Serbia, it is a rather small country so it is also possible that they have only high platforms, at least on the lines to be equipped with these DMUs.
You mean the rolling stock with adjustable gauge? Then it's those Talgo trains which are to run to EU. Or also we have dual-gauge TG16M locomotives (>>132173), but they are multi-gauge just technically, their running gear is designed for easy change between 1067 mm and 1520 mm wheelpairs because Sakhalin railway is in the middle of gauge alteration.