n. (context military English) a large mobile artillery gun mounted on a railroad rail car
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.