Marshall served as Fauquier County's delegate in the Virginia General Assembly 1836-1838, alongside Elias Edmunds in 1836-37 and alongside Edward Digges in 1838; the pair succeeded William R. Smith and Absalom Hickerson and was succeeded by Elias Edmonds and Josiah Tidball. Various Marshall family members had held one of the county's two seats in the House of Delegates for most of the decade. His elder brother Thomas Marshall had won election several times, and his younger brother James K. Marshall would twice win election. As did his brothers, E.C. Marshall farmed using enslaved labor. He owned 34 slaves in Fauquier County in 1850, and 38 slaves in Fauquier County's Southwest Revenue District in 1860. Two prewar riding accidents restricted this Marshall's mobility and caused him to use a cane or various contrivances-—the 1836 accident injured his ankle severely and the second accident caused 13 years of confinement, although E.C. Marshall was determined to remain active and traveled 6 miles to church to teach Sunday School. Marshall envisioned linking the farms of the Shenandoah Valley and his Piedmont region at the Manassas Gap with the port cities of Alexandria, Virginia and Richmond, Virginia. Marshall became the president of the Manassas Gap Railroad, which obtained a charter from the Virginia General Assembly in 1850. That year Marshall also sold the home constructed for him and his new wife after their wedding. The railway would link Strasburg, Virginia to Tudor Hall, where it joined with the Orange and Alexandria Railroad, which thus linked the Piedmont and Shenandoah Valley farmers to the ports of Alexandria, Virginia and Washington, D.C. as well as Richmond, Virginia. Construction began, and by 1854 the line extended from Mount Jackson, Virginia to Manassas. Towns also grew on the route, including Markham, Fauquier County, Virginia near Marshall's home. To reduce lease payments to the O&A, the MGRR began raising funds to construct an alternate line between Gainesville, Virginia and Alexandria. However, opposition of some landowners delayed construction, until the Panic of 1857 made financing difficult. During American Civil War, Confederate troops embarked in Delaplane to the First Battle of Manassas. Portions of the unfinished MGRR Independent Line also served as earthworks for Confederate troops at the Second Battle of Manassas. Both armies used the railroad to transport troops as well as supplies. Sections of the line were destroyed many times; none of its rolling stock survived the war. Furthermore, his son James Keith Marshall, an 1860 VMI graduate, was commissioned a Confederate officer and died at the Battle of Gettysburg. In 1867, Marshall sold the remaining Manassas Gap Railroad assets to the Orange and Alexandria Railroad, which completed and rebuilt it before being absorbed into the Richmond and Danville Railroad. After his death, it was absorbed into the Southern Railway, a multi-state system of over 3,000 miles. Meanwhile, Marshall secured a place in the Pension Office in Washington D.C. during the administrations of Ulysses S. Grant and Rutherford B. Hayes. He had built a new home, "Innis", in 1871-1872, and his family continued farming, and he visited during breaks from Washington, as well as also assisted in the nearby school run by his friend Dr. Jacquelin Ambler.
