Big gun (just finished firing) being covered up prior to moving
As shown in this image, some of the guns used during the conflict were so big that they could only be moved around on railway lines. The men in the photograph are unfolding the tarpaulin to cover the gun and protect it from any adverse weather conditions. It may also have been used to camouflage the gun from enemy reconnaissance.
Railway artillery was ideal for deployment during 'the Great War' because of the static nature of the fighting, and all the warring armies used such guns. At first, the railway artillery used by the British army was surplus naval artillery moved onto hydraulic railway wagons. The German army also used railway artillery on the Western Front. One of their most powerful rail guns was the famous 'Paris Gun', which shelled the capital from a distance of 130 kilometres.
[Original reads: 'OFFICIAL PHOTOGRAPH TAKEN ON THE BRITISH WESTERN FRONT. A big gun (just finished firing) being covered up prior to moving.']