Because of the established railroad infrastructure of Europe during World War 1 (1914-1918), the railway gun came into prominence as a viable wartime artillery measure. This class of weapon allowed for extreme-long-range delivery of very-heavy-caliber projectiles which caused massive damage on a target area - while also serving as something of a psychological tool against civilian populations. This sort of weapon was personified in the conflict by the "Paris Gun" fielded by the Germans from March 1918 onwards. The 256-ton weapon held a running length of 111 feet, 7 inches and fielded a barrel of 211mm diameter. While it used much important wartime material, man power, and time in its manufacture and operation, it no doubt forced the Parisian public to keep an ear to the sky as the French capital now lay within reach of this formidable German weapon.
Despite its obvious usefulness, the gun was not a perfected specimen. The firepower required of the system meant that fewer than fifty projectiles could be fired from the single barrel - such was the wear and tear of the action - for subsequent firing stripped some of the barrel lining away. This forced munitions makers to gradually increase the caliber of the projectiles in turn which was not a logistically sound solution. As such, the weapon played only a minor role in the closing months of the war which eventually saw Germany declared the loser and its war-making capabilities extremely limited heading forward.
During the interwar years, the Germans managed to evolve the railway gun idea only slightly but it was not until the Nazi party gained control of the government and its military that serious work on a modern railway weapon began. The charge once again fell to heavy industry specialist Krupp and the new weapon became the 21cm K12(E) (Kanone Model 12 "EIsenbahnlafette"). This gun system was also designed around a 211mm barrel but improvements were brought into its arrangement to help contend with the forces at play. Design work on the system spanned form 1935 to 1938 to which, in 1938, production managed the first of two weapons to be had.
As finalized, the 21cm K12 weighed in the 333-ton range with an overall length of 135 feet, 6 inches. The barrels alone measured 109 feet, 3 inches long. The breech mechanism was a horizontal sliding block arrangement with recoil handled through a hydropneumatic system. The barrel was fitted onto a specially designed structure which held an inherent elevation span of 25- to 55-degrees and 25-degrees of traversal. As a railway gun, the gun support structure was sat upon a special two-piece carriage which included a 10-axle component at front and an 8-axle component at rear to help provide the needed transport functionality while also supporting the massive weight of the gun and mounting hardware. The complex system required multiple crew for maximum performance - several dozen personnel at the very least. A hydraulic jack system was installed to elevate the gun some 3 feet above the ground.
The 211mm projectiles used separate-loading cased charges. The gun fired at a muzzle velocity of up to 5,400 feet per second with an effective range out to 49,120 yards. Maximum range was stated as high as 125,765 yards. Each shell weighed 237lbs so a special crane wagon was attached to the train. Due to the breech's location near the ground when the gun barrel was raised for firing, reloading was only possible by lowering the gun, giving the appropriate access to the breech.
In practice, the first received gun proved the concept sound though the Army did not appreciate the length of prep work required of the gun between firing actions. Hydropneumatic balance presses were developed by Krupp to remedy the problem and this produced the second weapon system under the designation of "K12N(E)". With its arrival, the first example became known under the "K12V(E)" designation.
The guns managed a certain level of limited service during World War 2 but weapons like this were generally limited by the established railway network of European countries and this quality and reach varied across the different players falling under German conquest. The weapons could rely on a special turntable emplacement if to be featured as a static defense gun. Regardless, they proved tactically inflexible and ended their days as static fixtures overlooking the English Channel at the French coast. Ultimately, the weapons proved useless once the Allies had made headway into France and German positions were overrun.