linguistic:translation, quality:correction
1860–1865: American Civil War

Before the American Civil War, Carnegie arranged a merger between Woodruff's company and that of George Pullman, the inventor of the sleeping car for first-class travel, which facilitated business travel at distances over 500 miles (800 km). The investment proved a success and a source of profit for Woodruff and Carnegie. The young Carnegie continued to work for Pennsylvania's Tom Scott and introduced several improvements in the service.

In the spring of 1861, Carnegie was appointed by Scott, who was now Assistant Secretary of War in charge of military transportation, as Superintendent of the Military Railways and the Union Government's telegraph lines in the East. Carnegie helped open the rail lines into Washington D.C. that the rebels had cut; he rode the locomotive pulling the first brigade of Union troops to reach Washington D.C. Following the defeat of Union forces at Bull Run, he personally supervised the transportation of the defeated forces. Under his organization, the telegraph service rendered efficient service to the Union cause and significantly assisted in the eventual victory. Carnegie later joked that he was "the first casualty of the war" when he gained a scar on his cheek from freeing a trapped telegraph wire.

The defeat of the Confederacy required vast supplies of munitions, railroads and telegraph lines to deliver the goods. The war demonstrated how integral the industries were to Union success.