The British Ordnance BL 12 inch gun Mk IX on truck, railway mounted surplus 12 inch Mk IX W naval guns, manufactured by Woolwich Arsenal in 1906,[2][note 1] on various railway platforms to provide mobile long-range heavy artillery for the British Army on the Western Front in World War I. Vickers mounted two Mk IX W guns on slightly different railway mountings, Mk I, from September 1915.

The gun cars allowed only 1° left and right traverse, achieved by pivoting the entire gun car body about the forward bogies, in the two Vickers carriages, or about the rear bogies in the Armstrong carriages.

Like most railway guns in the war it was operated on specially-constructed curved sections of track and moved forward or backward to point it at a new target.

Fine adjustment could then be achieved by onboard traversing, which at the gun's maximum range covered an arc of about 1,000 yards (910 m).

[5] The initial shock of firing was absorbed by a hydro-spring mechanism, allowing 34 inches (86 cm) of recoil within the gun mounting.