The six remaining companies went on a three-day exercise march alongside artillery on 22 September. The regiment began moving to Pittsburgh on 28 September, but when it reached Huntingdon the order was changed. The regiment instead entrained aboard the Washington and Alexandria Railroad to Manassas Junction, serving along the Manassas Gap Railroad with its headquarters at Gainesville. It later moved to Thoroughfare Gap, remaining there until the rail was broken up. This duty was to protect railroad work crews from being attacked by the Confederate partisan rangers of John Mosby.