The Gun « Dora » :
(The 80cm L/40.6 (E) "Dora" was designed and built by Krupp)

Germans made a speciality out of their railguns since the first world war conflict. Let's think about that railgun which terrorised Paris in 1918, if he made lightfull casualty it was a psychologic heavy weapon, by showing the capacity of the German army in striking anywhere in the capital. But the primary interest of the railgun was that the Germans could carry lots of heavy ammunitions and also heavy weapons very close to the battlefields.
At this time, roads weren't really developed in Germany and Western Europe so they couldn't allow the carrying of tons of ammunition and not even the weight of «Dora», the heavy gun that could weigh hundreds of tons.
The different gauges beetween rails in Russia, Poland and Germany was a real problem for railguns and so they often had to change the axle or even build new railways.

Obviously those guns were made to lay siege to a town or blowing out allied troups nest or in a situation of trench war on the end of the first Worldwar. Some of the 280 mm's were used at the Pas de Calais (France) to shell England.
The most famous, but also the less used was «Dora», with an incredible caliber of 800 mm (it means that the diameter of the gun was 80 cm !!!). It was the biggest caliber ever used in the history of artillery. The shells weighed 4,8 kilos (and up to 7 for the perforating ones). The useful range was about 48 km (some which where aiming england could shoot from 120 km).

Commanded by Hitler since 1937 at the Krupp factories,the canon was made to pulverised the Maginot line, None defensive post, concrete-made and underground ones too, could resist to a direct shelling. To use that gun it needed at least 5000 people (500 gunners, and the other for the logistic transport and also protection) and six weeks of preparation were needed to use it. «Dora» could spit a shell every fifteen minutes a rythm rarely reached.

That gun was build in two exemplaries, a third one not completed («Langer Gustav» and a fourth called «schwerer Langer Gustav» was in expectation) but only one was used.
«Schwerer Gustav» was completed in 1941 but to late for the french campaign. His short carrier is summed up at the Sebastopol siege.
To heavy, to slow, it was captured by the Soviets, during the German retreat in 1944, probably in Crimea.
The incomplete «Langer Gustav» was destroyed by air attacks and «Dora» and «Schwerer Gustav» were blown up by a German blowing up command in Kummersdorf in 1945.