The 18 inch Railway Howitzer was suggested in 1917 as a partner to the 14 inch Railway Gun using the same carriage. There were no existing guns of that calibre, so a new gun was designed. It was to have the same profile and size of the 14 inch Gun so that it could fit the 14 inch’s carriage without modification. Despite the difference in size, this was possible because the stresses on a howitzer barrel were much less than on a gun and therefoe allowed the barrel walls to be thinner. The design was finished in 1918 and two barrels were made. They were not completed in time for the war and so were mounted, proofed and then put in store.
In 1926 the 14 inch was made obsolete and scrapped but the carriages were retained. One of the 18 inch barrels was mounted on HMG Boche-Buster and taken to Salisbury Plain. During the inter-war years the gun was fired twice. In 1940 it was sent to Dover to become part of the invasion defences at Bekesbourne on the Dover/Canterbury line. Being a howitzer it barely had the range to cover the beaches of the Kent coast. It was never fired in anger and doubtful if it fired much in training either. In 1943 it was back on Salisbury Plain firing trials of a new anti-concrete shell. This is probably the last time it ever fired.
Late in 1943 it joined the 13.5 inch guns in a Super Heavy Railway Regiment. The idea of using the regiment in the invasion was soon dropped and the regiment disbanded. The 18 inch Railway Howitzer was declared obsolete in April 1947 and later scrapped.
Data
| Weight of gun & breech mechanism | 191,968 lbs (85 tons) |
| Total length | 648.4 inches |
| Length of Bore | 624.6 inches (35 calibres) |
| Rifling | 72 grooves, uniform Right Hand 1/20 |
| Breech mechanism | Interrupted screw, continuous motion, electric & percussion fired |
| Elevation | 0º to +40º |
| Traverse | 2º right & left |
| Recoil system | Hydropneumatic, constant 34 inches |
| Weight in action | 560,935 lbs (250 tons) |
Performance
Firing standard 2,500 lb HE Shell
| Muzzle velocity | 1,880 feet per second |
| Maximum range | 22,300 yards |
Ammunition
Separate loading, bag charge
| Shell, HE Mark 2D | Streamlined, nose fuzed shell weighing 2,500 lbs. Fitted with Percussion Fuze no 231 |
| Shell, CP, BC, S/L Mark 1D | Common Pointed shell with Ballistic Cap & Streamlined base. Fitted with Base Percussion Fuze No 270 |
| Shell Anti-Concrete S/L Mark 1D | Approved 1944, an advanced design. Concave head under a ballistic cap. Fitted with Base Percussion Fuze No 270 |
| Propelling charge | 7 charge options in 2 portions, long & short range. Charge 1 Short range + increments 2, 3 & 4 total weight 182 lbs of Cordite. Charge 5 Long range + increments 6 & 7 total weight 282 lbs of Cordite |