The railroad industry or field was the single largest business in the United States by 1860, even twice the size of military (comparing officers to railroad workers).
There were 16,215 officers in the United States Army by 1860 (10,929 in 1850). The number of men working in the railroad industry was twice the size of military officers in 1860, with 36,567 men working in the railroad industry. From 1850 until 1860 the number of men working in the railroad field increased by 900%.
Source for info: Railroads and the Civil War, Clark, p. 9.
Image source: Ruins of the Confederate enginehouse at Atlanta, Ga., September 1864, showing the engines “Telegraph” and “O.A. Bull.” 111-B-4748. National Archives Identifier: 528865