In 1917, railway-mounted artillery was not a new innovation; the first recorded use of cannon mounted on railcars was during the Civil War. World War I, however, as it did with many other innovations, expanded the combination’s scope and breadth.

The Germans first used very large naval guns at Verdun in 1915. The eight 38-centimeter (15-inch), 45-caliber guns were designed and built as the main armament for two stillborn battleships of the Bayern class. As first employed, they required fixed concrete emplacements that took months to build. In November 1917, the U.S. Navy’s Bureau of Ordnance received a report from Navy Lieutenant Commander G. L. Schuyler with information concerning the maximum range of one of the guns, singly emplaced near Ostend, Belgium, which was firing into Dunkirk. Known as the “Leugenboom” gun, it had a range of 50,300 yards, or slightly more than 28.5 miles. No British guns in the sector could equal this range, and it was evident that the Germans were making great strides in modifying their naval guns so that they could be used on land for long-range bombardment.

An Idea Born

https://www.usni.org/magazines/naval-history-magazine/2021/october/our-highest-hope-was-gained-naval-artillery-ashore?mc_cid=d774abc702&mc_eid=770462a2eb