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.