its operations at Dallas Love Field. Texas's second-largest air facility is Houston's George Bush Intercontinental Airport (IAH). It served as the largest hub for the former Continental Airlines, which was based in Houston; it serves as the largest hub for United Airlines, the world's third-largest airline, by passenger-miles flown. IAH offers service to the most Mexican destinations of any U.S. airport. The next five largest airports in the state all serve more than three million passengers annually; they include Austin-Bergstrom International Airport, William P. Hobby Airport, San Antonio International Airport, Dallas Love Field and El Paso International Airport. The smallest airport in the state to be designated an international airport is Del Rio International Airport. Ports Around 1,150 seaports dot Texas's coast with over of channels. Ports employ nearly one-million people and handle an average of 317 million metric tons. Texas ports connect with the rest of the U.S. Atlantic seaboard with the Gulf section of the Intracoastal Waterway. The Port of Houston today is the busiest port in the United States in foreign tonnage, second in overall tonnage, and tenth worldwide in tonnage. The Houston Ship Channel spans wide by deep by long. Railroads Part of the state's tradition of cowboys is derived from the massive cattle drives which its ranchers organized in the nineteenth century to drive livestock to railroads and markets in Kansas, for shipment to the east. Towns along the way, such as Baxter Springs, the first cow town in Kansas, developed to handle the seasonal workers and tens of thousands of head of cattle being driven. The first railroad to operate in Texas was the Buffalo Bayou, Brazos and Colorado Railway, opening in August 1853. The first railroad to enter Texas from the north, completed in 1872, was the . With increasing railroad access, the ranchers did not have to take their livestock up to the Midwest and shipped beef out from Texas. This caused a decline in the economies of the cow towns. Since 1911, Texas has led the nation in length of railroad miles within the state. Texas railway length peaked in 1932 at , but declined to by 2000. While the Railroad Commission of Texas originally regulated state railroads, in 2005 the state reassigned these duties to TxDOT. In the Dallas–Fort Worth area, three public transit agencies provide rail service: Dallas Area Rapid Transit (DART), Denton County Transportation Authority (DCTA), and Trinity