rivers complete the commercial transportation modes for West Virginia. Commercial air travel is facilitated by airports in Charleston, Huntington, Morgantown, Beckley, Lewisburg, Clarksburg, and Parkersburg. All but Charleston and Huntington are subsidized by the federal Department of Transportation's Essential Air Service program. The cities of Charleston, Huntington, Beckley, Wheeling, Morgantown, Clarksburg, Parkersburg and Fairmont have bus-based public transit systems. West Virginia University in Morgantown boasts the PRT (personal rapid transit) system, the state's only single-rail public transit system. Developed by Boeing, the WVU School of Engineering and the Department of Transportation, it was a model for low-capacity light transport designed for smaller cities. Recreational transportation opportunities abound in West Virginia, including hiking trails, rail trails, ATV off-road trails, white water rafting rivers, and two tourist railroads, the Cass Scenic Railroad and the Potomac Eagle Scenic Railroad. West Virginia is crossed by seven Interstate Highways. I-64 enters the state near White Sulphur Springs in the mountainous east, and exits for Kentucky in the west, near Huntington. I-77 enters from Virginia in the south, near Bluefield. It runs north past Parkersburg before it crosses into Ohio. I-64 and I-77 between Charleston and Beckley are merged as toll road known as the West Virginia Turnpike, which continues as I-77 alone from Beckley to Princeton. It was constructed beginning in 1952 as a two-lane road, but rebuilt beginning in 1974 to Interstate standards. Today almost nothing of the original construction remains. I-68's western terminus is in Morgantown. From there it runs east into Maryland. At the I-68 terminus in Morgantown, it meets I-79, which enters from Pennsylvania and runs through the state to its southern terminus in Charleston. I-70 briefly runs through West Virginia, crossing the northern panhandle through Wheeling, while I-470 is a bypass of Wheeling (making Wheeling among the smallest cities with an interstate bypass). I-81 also briefly runs in West Virginia through the Eastern Panhandle where it goes through Martinsburg. The interstates are supplemented by roads constructed under the Appalachian Corridor system. Four Corridors are complete. Corridor D, carrying US 50, runs from the Ohio River, and I-77, at Parkersburg to I-79 at Clarksburg. Corridor G, carrying US 119, runs from Charleston to the Kentucky border at Williamson. Corridor L, carrying US 19, runs from the Turnpike at Beckley to I-79 near Sutton (and provides a short cut of about and bypasses Charleston's urban traffic for traveler heading to and from