This picture shows American troops as they load a massive rail gun.
Rail guns were moderately used in World War Two, and much more heavily utilized in the First World War.
The idea of a railway gun is to be able to “quickly” move massive artillery along rail tracks to a position where it can pummel the enemy with immensely large shells.
Examples you probably know are the German Dora and Gustav guns, both insanely large.
The problem with rail guns as large as the Gustav Gun though, is that that in a mobile war like World War Two was, it becomes hard to effectively use the gun as the enemy usually is never in the same place long. That’s why they are usually used in sieges, like during the Siege of Sevastopol, where the Gustav shot multiple shells into the city, one of which traveled a good ways underground and detonated in a hidden Soviet bunker.
In World War One, the idea of a railway gun was much more effective and practical, especially on the Western Front. With static trench lines running from Belgium down towards Switzerland, a railway gun would have been devastating as the enemy did not move much, and it could be moved up and down the line on its rails to respond to certain enemy troop or, later, tank concentrations.
But, in World War Two, railway guns didn’t do a whole lot, and personally I would have used the recourses put into building more tanks or aircraft, which really one the war.