"The design of the corps," wrote McCallum, in his final report, "was to combine a body of skilled workmen in each department of railroad construction and repairs, under competent engineers, supplied with abundant materials, tools and mechanical appliances." The corps was formed into divisions the number of which varied from time to time, in different districts, according to requirements. In the military division of the Mississippi the corps comprised six divisions, under the general charge of the chief engineer of the United States military railroads for that military division, and consisted at its maximum strength of nearly 5,000 men. In order to give the corps entire mobility, and to enable it to move independently and undertake work at widely different points, each of the six divisions was made a complete unit, under the command of a divisional engineer, and was, in turn, divided into sub-divisions or sections, with a supervisor in charge of each. The two largest and most important sub-divisions in any one division were those of the track-layers and the bridge-builders. A sub-division was, again, composed of gangs, each with a foreman, while the gangs were divided into squads, each with a sub-foreman.[6] Under this method of organisation it was possible to move either the entire division or any section thereof, with its tools, camp requirements and field transport, in any direction, wherever and whenever needed, and by any mode of conveyance--rail, road, with teams and wagons, or on foot. To facilitate the operations of the corps, supplies of materials were kept at points along or within a short distance of the railway lines, where they would be comparatively safe and speedily procurable in case of necessity. At places where there was special need for taking precautionary measures, detachments of the corps were stationed in readiness for immediate action, while on important lines of railway Federals and Confederates alike had, at each end thereof, construction trains loaded with every possible requisite, the locomotives attached to them keeping their steam up in order that the trains could be started off instantly on the receipt of a telegram announcing a further interruption of traffic. At Nashville and Chattanooga the Federals built extensive storehouses where they kept on hand supplies of materials for the prompt carrying out of railway repairs of every kind to any extent and in whatever direction. On the Nashville and Chattanooga Railway itself the Construction Corps, from February, 1864, to the close of the war, relaid 115 miles of track, put in nineteen miles of new sidings, eight miles apart and each capable of holding from five to eight long freight trains, and erected forty-five new water tanks.