Panzerjager-Treibwagen means the tank destroyer on the rail. It had two long-barrel turret, which came from German Panzer IV tank. The train was designed according to soviet MBV-D4. Production since December, 1944. Only 3 of 5 were completed.
I don't understand why not. Looking at blocky, jagged planes and multicolored tanks gets old after awhile, so you'd think people would want to explore new concepts. Their loss, I guess. Plus, the blocky, rectangular aesthetic of an armored train translates well into lego. If I can dig up my railway sets from my basement, I might have to try a few my self.