Who am I? - going with John W Garrett. Lincoln called him the "right arm of the Federal Government"
What was my Civil War service? - Amongst other things he provided Union troops with intelligence on Lee's movements before Gettysburg - also helped save Washington from General Early's attack.
Edit - I will accept Garrett, who became president of the Baltimore & Ohio in 1858, as a correct answer to the question.
1) Andrew Carnegie
2) 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. Defeat of the Confederacy required vast supplies of munitions, as well as railroads (and telegraph lines) to deliver the goods.
During the Civil War he was Assistant Secretary of War, superintendent of the Military Railways and controlled the Government's telegraph lines - vital to the war effort. "He ensured that rails were opened to Washington, DC and that defeated troops from the Battle of Bull Run were removed safely."
His name was known in communities all over America because of the libraries he had built with his money. https://thevintagenews.com/2017/02/...d-his-more-imperative-role-as-a-philantropist
Edit - I am accepting either Carnegie's work with the military railways or his work with the telegraph system as a correct answer to the second part of the question.
If the Encyclopedia Britannica article provided in support of the official answer makes any mention of Carnegie's service during the Civil War, I wasn't able to find it, but other sources submitted by the players make it clear that he was the best answer to the question.