Wikipedia:
A railway gun, also called a railroad gun, is a large artillery piece, often surplus naval artillery, mounted on, transported by, and fired from a specially designed railway wagon. Many countries have built railway guns, but the best known are the large Krupp-built pieces used by Germany in World War I and World War II. Smaller guns were often part of an armoured train.
Railway guns have been rendered obsolete by advances in technology. Their large size and limited mobility make them vulnerable to attack, and similar payloads can be delivered by aircraft, rocket, or missile.
The outbreak of the First World War caught the French with a shortage of heavy field artillery. In compensation, large numbers of large static coastal defence guns and naval guns were moved to the front, but these were typically unsuitable for field use and required some kind of mounting. The railway gun provided the obvious solution. By 1916, both sides were deploying railway guns.