By Colin Salt
World War II Germany is infamous for producing a ton of paper wunderwaffe that its severely resource-constrained, factionalized economy could never actually support. One of the less familiar but unsurprising products was a plan to make railway guns more mobile by attaching them to a pair of tanks. The resulting monster is so "World War II Germany" that it's honestly a little weird that it hasn't attracted more public attention.
The "What" of the giant gun attached to two giant tanks varied, but one thing that's interesting is that the "How" is a lot less ambiguous. In fact, the role of these giant cannons (and later, rockets/missiles) has remained virtually unchanged since World War I. The technology is obviously different. The nature of calling for their support is also obviously changed. But what they're supposed to do has stayed remarkably similar. There are three main tasks that have been assigned to heavy artillery, almost always controlled at echelons above division.
The first is counter-battery, or engaging enemy artillery. Here, the longer range of most systems of this nature has an obvious advantage. The second, also reliant on greater range, is deep strike. It needs no explanation why you'd want something that can hit targets that regular "general support" artillery can't. The third is less about range (although that always helps) and more about power. That is the ability to "reinforce" lower formations by firing at targets in their sectors. For instance, the big corps+ level guns/missiles would be used to either aid or stop a breakthrough.
An American set of field regulations from 1923 sums up the role of the big-unit big gun. “The primary mission of corps artillery is the destruction or neutralization of hostile batteries, the destruction of hostile defenses, and long-range interdiction fire.” While there have always been variations in the exact makeup, nearly every army with such weapons has used them in this way for over a century.
So the railway-tank guns would stand alongside both the converted naval pieces of the previous World War that was and the M110s and 2S7s of the next World War that wasn't. As would any other mega-gun of the 20th Century. When I was younger, I whimsically wondered how you'd use the 16 inch gun from an Iowa on a tracked chassis. Now I learn that a similar weapon was made (and quickly discarded as impractical), and I don't have to wonder anymore as to how such a beast would be deployed.