In 1840 Kirkwood was Resident Engineer on the Mountain Division of the Western Rail Road., where he remained until its completion in 1843. He located and constructed the Springfield & Northampton Rail Road. In 1848 he completed the Starrucca Viaduct as Superintendent in one season; The Starrucca Viaduct near Lanesboro, Pennsylvania, considered to be the most expensive railroad bridge at the time, as well as the largest stone viaduct, and for its first use of concrete in American bridge construction. This success led to his appointment as General Superintendent of the Erie Railroad on April 1, 1849, where he succeeded Hezekiah C. Seymour.