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. They were 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.
Design considerations The design of a railway gun has three firing issues over and above those of an ordinary artillery piece to consider. Namely how the gun is going to be traversed – i.e. moved from side to side to aim; how the horizontal component of the recoil force will be absorbed by the gun's carriage and how the vertical recoil force will be absorbed by the ground.
Methods of traverse
The first method of traverse is to rely entirely on movement along a curved section of track or on a turntable with no provision to traverse the gun on its mount. The second is to traverse the rail car body on its trucks, known as a car-traversing mount. Generally this is limited to a few degrees of traverse to either side unless an elaborate foundation is built with a centre pivot and traversing rollers. The design of the foundation is the only limit to the amount of traverse allowed in this latter case. The third choice is to allow the separate gun mount to rotate with respect to the rail car body, known as a top-carriage traversing mount. This usually requires the gun to be mounted on a central pivot which, in turn, is mounted on the car body. With few exceptions these types of mounts require some number of outriggers, stabilisers, or earth anchors to keep them in place against the recoil forces and are generally more suitable for smaller guns. The American post–World War I assessment of railway artillery considered that the utility of even a small amount of traverse for fine adjustments was high enough that either of the two latter traversing methods is preferable to a fixed mount.
Recoil systems
There are four primary methods to absorb the recoil force for railway guns: cradle recoil, top-carriage recoil, sliding recoil and rolling recoil. Cradle recoil means that the gun recoils backward in its cradle, slowed and stopped by hydraulic buffers. It is returned to battery, or the firing position, by either helical springs or by air in a pneumatic recuperator cylinder that is compressed by the force of recoil. This is the most common method used for lighter railway guns and for virtually all field artillery designed after the French introduced their Canon de 75 modèle 1897. Top-carriage recoil is the situation in which the gun is mounted in an upper carriage that moves on wheels on fixed rails mounted on the lower. The gun and upper carriage recoil together, restrained by the usual hydraulic buffers. Return to battery is effected either by gravity, through the use of inclined rails, which the gun and carriage have run up, by springs, or even by rubber bands, on some improvised mounts. It is not well-suited to firing at steep upward angles because it cannot absorb much of the vertical component of the recoil force. Sliding recoil has the car body sitting on a set of wooden crossbeams or "sleepers" placed underneath it which have been jacked down on to a special set of girders incorporated into the track so that about half the weight of the mount has been transferred to them from the trucks. The gun, car body and trucks all recoil together with the friction generated by the crossbeams sliding on the girders absorbing the recoil force after moving only about 1 to 2 metres (3.3 to 6.6 ft) to the rear. The sleepers must be jacked up again to allow the gun to roll forward to its firing position. This was often done by handwheels driving gear trains attached to the wheels, or even by electric motors on more modern mounts. Almost all of these types of mounts were of the non-traversing type and had to be fired from a curved section of track or turntable. The American post–World War I assessment of railway artillery praised its ruggedness, ease of manufacture and convenience in service, but acknowledged its unsuitability for smaller guns, due to excessive time of operation and lack of traverse, and that it was not suitable for the largest howitzers firing at high angles because of the enormous trunnion forces. With rolling recoil the entire gun, mount, and carriage rolls backward, typically between 30 and 50 feet (9.1 and 15.2 m), restrained only by the brakes. The mount was winched back into firing position by cables fastened to the track. This system was usually combined with cradle recoil because the springs of the trucks cannot withstand the vertical component of the recoil force alone. This type of mount was usually fitted with car-traverse. It was unsuitable for smaller guns due to the lack of traverse. The great advantage of this method is that it requires minimal preparation and can fire from any suitable section of curved track. The methods were often used in combination with each other. Examples include the French 520 mm (20 in) railway howitzer which used cradle-sliding recoil. The American 14"/50 caliber railway gun Mark II used cradle-rolling recoil as did the 14 and 12 inch railway guns from Great Britain. Only the oldest weapons used a combination of top-cradle and sliding recoil. One example being the earliest mounts for the British designed BL 9.2 inch Railway Gun.
Anchorage
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