Every Corps must be based upon a thoroughly reliable and efficient line of supply, and for this a railway in first-class operating condition is a prime essential. Every kind of requisite must be carried by rail to some advanced distribution point called a "railhead." Thence supplies are distributed by motor lorry to the areas still further forward.
The appropriate distance of the railhead behind the battle front is conditioned by the available supply of motor lorries, and their range of action. If the distance be too great the stress upon the mechanical transport becomes so severe that it rapidly deteriorates, and an undue proportion of lorries daily falls out of service. As the facilities for repair in the mobile workshops are strictly limited, an excessive rate of wastage among these vehicles soon dislocates the whole supply arrangements.
The experience hitherto gained had demonstrated that a railhead could not conveniently be allowed to fall behind our advance more than ten or twelve miles. This limit had already been reached when the Corps front arrived on the west bank of the Somme, and the strain upon the lorry service was already great.
For a further deep advance of the whole Corps in pursuit of the enemy towards the Hindenburg Line, still distant another fifteen miles, it became imperative, therefore, that the railway service to Péronne and beyond should be speedily reopened, or some equally efficient alternative provided. The great lattice girder railway bridge at Péronne had been irretrievably demolished. Engineers estimated that it would take two months to restore it, and at least a month to provide even a temporary deviation and crossing. Nevertheless, the work was put in hand without delay.
An alternative possibility was to construct a new line of railway to connect the existing military line at Bray to the Péronne railway station, a length of new construction amounting to some six miles. It was estimated that such a link could be built in a fortnight, and this work also was commenced forthwith.
There was a third possibility. This was speedily to repair that portion of the railway which lay west of the Somme, and to establish a railhead near Péronne, but on the opposite bank of the river. This proposal involved only a few days' work, for extensive sidings already existed on the west bank, and had been left more or less undamaged by the enemy. But it also involved the complete restoration of all road traffic bridges, both at Péronne and at Brie, for the service of the intense traffic which would ensue across the Somme from such a point of departure.
The rebuilding of the crossings was, in any case, a matter of urgent necessity. By this time all my heaviest guns had already been brought up to the vicinity of the west bank of the Somme, and had there perforce to wait; for a long detour, on the densely-crowded roads, to cross the Somme, say as far back as Corbie, where bridges were strong and grades were easy, was out of the question.
The problem, therefore, involved a stable and comprehensive reconstruction; half measures would not meet the case. But half measures were an inevitable necessity of the situation, to begin with, because troops had to be fed, and their supplies could be carried in no lighter way, in adequate quantities, than in the normal horse-transport wagons.
The order of procedure had, therefore, to be, firstly, hastily to reconstruct some sort of bridging, based generally upon the wreckage of the original bridge, and strong enough to carry loads up to those of horsed wagons; next to stay, strut and strengthen these temporary bridges to fit them for the passage of the lighter guns, and finally to reconstruct them in their entirety for the heaviest loads.