On May 28, 1862, Haupt was appointed Chief of Construction and Transportation in the Department of the Rappahannock, with the rank of Colonel. He was raised to the rank of Brigadier-General in the following year, and did much excellent construction and other work for the Government, though mainly in Virginia, down to September, 1863. In his "Reminiscences" he relates that the supplies of repair or reconstruction materials, as kept on hand by the Federals, included the interchangeable parts of bridge trusses, in spans of 60 ft., and so prepared that, taken on flat cars, by ox-teams or otherwise, to the place where they were wanted, and hoisted into position by machinery arranged for the purpose, they could, without previous fitting, be put together with such rapidity that one of his foremen claimed to be able to build a bridge "about as fast as a dog could trot." When the Massaponix bridge, six miles from Fredericksburg, was burned down one Monday morning, a new one was put up in its place in half a day—a feat which, he says, led some of the onlookers to exclaim, "The Yankees can build bridges quicker than the Rebs can burn them down." In May, 1862, five bridges over Goose Creek which the "Rebs" had destroyed were reconstructed in a day and a half. In the following month five other bridges, each with a span of from 60 ft. to 120 ft., were renewed in one day. At the Battle of Gettysburg Lee's troops destroyed nineteen bridges on the Northern Central Railroad and did much havoc on the branch lines leading to Gettysburg; but the Construction Corps was hard at work on the repairs whilst the battle was still being waged, and rail communication with both Washington and Baltimore had been re-established by noon of the day after Lee's retreat.
In some instances railway bridges underwent repeated destruction and reconstruction. By June, 1863, the bridge over Bull Run, for instance, had been burned down and built up again no fewer than seven times. Many of the bridges, also, were swept away by floods, and this even for a second or a third time after they had been rebuilt. Precautions thus had to be taken against the destructive forces of Nature no less than against those of man.
Haupt's pioneer Construction Corps in Virginia was succeeded by the one set up on much broader lines by McCallum when, in February, 1864, he became General Manager of railways in the Military Division of the Mississippi. This corps eventually reached a total of 10,000 men.
"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.
The reconstruction of this particular line was more especially needed in connection with General Sherman's campaign in Georgia and the Carolinas—a campaign which afforded the greatest and most direct evidence up to that time alike of the possibilities of rail-power in warfare, of the risks by which its use was attended, and of the success with which those risks could be overcome by means of efficient organisation.