Louisville is the largest city in the Commonwealth of Kentucky and the state's only 1st-class city. In 2010, Louisville proper was the 27th-largest city in the United States. Located beside the Falls of the Ohio, the only major obstruction to river traffic between the Ohio River and the Gulf of Mexico, Louisville first grew as portage site. The city was the headquarters of the Louisville and Nashville Railroad, which grew into a 6,000-mile (9,700 km) system across 13 states. Today, Louisville is best known as the location of the Kentucky Derby, the first of the three annual races that make up the Triple Crown of Thoroughbred Racing. It is the home of the University of Louisville and three of Kentucky's six Fortune 500 companies. Its airport is also the site of UPS's worldwide air hub.