currently one of five states with no transportation-related debts. Roads Tennessee has of roads, of which are maintained by the state. Of the state's highways, are part of the Interstate Highway System. Tennessee has no tolled roads or bridges but has the sixth-highest mileage of high-occupancy vehicle (HOV) lanes, which are utilized on freeways in the congestion-prone Nashville and Memphis metropolitan areas. Interstate 40 (I-40) is the longest Interstate Highway in Tennessee, traversing the state for . Known as "Tennessee's Main Street", I-40 serves the major cities of Memphis, Nashville, and Knoxville, and throughout its entire length in Tennessee, one can observe the diversity of the state's geography and landforms. I-40's branch interstates include I-240 in Memphis; I-440 in Nashville; I-840 around Nashville; I-140 from Knoxville to Maryville; and I-640 in Knoxville. In a north–south orientation, from west to east, are interstates 55, which serves Memphis; 65, which passes through Nashville; 75, which serves Chattanooga and Knoxville; and 81, which begins east of Knoxville, and serves Bristol to the northeast. I-24 is an east–west interstate that enters the state in Clarksville, passes through Nashville, and terminates in Chattanooga. I-26, although technically an east–west interstate, begins in Kingsport and runs southwardly through Johnson City before exiting into North Carolina. I-155 is a branch route of I-55 that serves the northwestern part of the state. I-275 is a short spur route in Knoxville. I-269 runs from Millington to Collierville, serving as an outer bypass of Memphis. Airports Major airports in Tennessee include Nashville International Airport (BNA), Memphis International Airport (MEM), McGhee Tyson Airport (TYS) outside of Knoxville, Chattanooga Metropolitan Airport (CHA), Tri-Cities Regional Airport (TRI) in Blountville, and McKellar-Sipes Regional Airport (MKL) in Jackson. Because Memphis International Airport is the hub of FedEx Corporation, it is the world's busiest cargo airport. The state also has 74 general aviation airports and 148 heliports. Railroads For passenger rail service, Memphis and Newbern are served by the Amtrak City of New Orleans line on its run between Chicago and New Orleans. Nashville is served by the Music City Star commuter rail service. Tennessee currently has of freight trackage in operation, most of which are owned by CSX Transportation. Norfolk Southern Railway also operates lines in East and southwestern Tennessee. BNSF operates a major intermodal facility in Memphis. Waterways Tennessee has a total of of navigable waterways, the 11th highest in the nation. These include the