The BL 18-inch railway howitzer was a British railway gun developed during World War I. Part of the progression of ever-larger howitzers on the Western Front, it did not enter service until 1920.

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. Only able to be moved where there were good tracks, which could be destroyed by artillery bombardment or airstrike, railway guns were phased out after World War II.