The Railway Gun
The absolute largest artillery piece mankind ever produced at the time, this is the legendary railway gun. The power of German engineering exemplified. As WWII was in its final stages, the German army needed weapons that were able to stop the encroaching Allied Forces and be able to continue their retreat at the time. Thus, the idea came to assemble this gargantuan canon. More or less, Germany was a highly industrialized nation, with much of its transportation routes relying on railway travel. Thus, being able to mount an artillery piece upon the rails would mean excellent transportability for this weapon throughout most of Europe.
The gun was built to be able to rain down massive payloads from a safe distance and was set to be the pride of the German Army. Unfortunately, its design was what truly hampered it on the battlefield. The barrel of the can was unable to rotate, so adjusting it was nearly impossible. You just kind of had to line it up on the tracks and pray the shells would reach the enemy.
Then there was the fact that it took a very long time to actually get the engine started and moving in case of a hasty retreat. Finally, the canon had a very low rate of fire because of how heavy its shells were. While railway guns did see combat, they were scrapped because of just how impractical they were.