 Between those two cities, US 11 follows the Susquehanna River, while I-81 follows a shorter route over the mountains.US 11 enters Pennsylvania parallel to Interstate 81 south of Greencastle in Antrim Township, Franklin County.The two routes head to the northeast, running parallel to each other as they pass through Chambersburg, where they intersect U.S. Route 30, Shippensburg, and Carlisle.