The attack begun, and the advance progressing, the railway detachment followed the front line closely, laying their tracks almost as fast as the infantry could push ahead. In this way supplies of provisions and ammunition were carried forward, while the wounded were carried back to the clearing hospitals. Within a week before the Arras offensive tracks had been laid to the top of Vimy Ridge, and by the end of April, 1917, when the British lines were pushed across the level plain beyond the Ridge, the light railways had followed them so closely that food supplies were dumped almost by the field kitchens. Such similar service was rendered by the Canadian Railway Construction Troops at Messines as well. It was at Ypres, however, that they especially distinguished themselves. During two months of the summer of 1917, says the official report, the average daily number of breaks in the light railway lines behind the front, due to German artillery fire, was about a hundred exclusively within the area occupied by the Second and Fifth British Armies alone. Here the Canadians pursued their construction work exposed to the full fire of the enemy guns, without even the moral satisfaction of being able to return the fire. On one occasion, however, they were to have this satisfaction in full. It was during the last four days of March, 1918, while the Germans were advancing on Amiens, that a break suddenly developed in the British lines. No reserves were available at the time. On the spur of the moment the railway men organized sixteen Lewis-gun teams and held the ground in the break until finally they were relieved by regular troops. Early in 1918 the Canadian Overseas Railway Construction Corps, the 58th Broad Gauge Operating Company, the 13th Light Railway Operating Company, the 69th Wagon Erecting Company, and the 85th Engine Crew Company were brought under headquarters, and the whole were formed into the Corps of Canadian Railway Troops. In the summer of 1918, General Allenby, in command of the expeditionary force in Palestine, called for a company of expert bridge builders. The War Office immediately called for volunteers from among the Canadian Railway Troops, and 6 officers and 250 men were sent to Palestine. The following table, taken from the report of the Ministry of the Overseas Military Forces of Canada, shows the relative strength of the Canadian and the Imperial Railway Construction Corps at different periods of the war: Nominal Strength Nominal Strength Imperial Railway Canadian Railway Construction Construction Troops Troops