The Baldwin Locomotive works built a number of 14-inch railway guns for the US Navy during WWI. They built them, outfitted entire trains to support the train/track and gun crews to be self-supporting with sleeping and eating quarters in about three months. They were shipped over and put to use right away.
Here is some footage of the guns being put together and during crew training:
A very interesting overview of them being set up and used in France, including footage of locomotives spotting the guns.
A surviving gun is at the museum at the Washington Naval Shipyard in Washington DC. There are less than a dozen or so railway guns survivors worldwide. The US Army captured one or more of the German guns in Italy (where the Germans "hid" the guns in tunnels between shots), putting one together and brought to the US for testing. This gun was moved (by rail) a few years ago from Maryland to Virginia when the Army shutdown the museum at Aberdeen.
Pre-WWII, the Army (?) operated a number of large guns as coastal defense along the West Coast. I understand they maintained a number of spurs along the Southern Pacific where the RR would move the guns to where they would be needed.
In the book Band of Brothers, the author described the unit followed in the book coming under fire from one of the German guns. They considered the guns to be more of an annoyance then anything because you could hear the shell coming with enough time to seek cover.
Certainly a US prototype would be very nice!
Bob