Harris County is the largest of the 11 counties in the Greater Houston region. Almost three-quarters of the county are covered by the city of Houston and thirty smaller communities; only about 27 percent of the county is rural. The county comprises 1,778 square miles and is the largest Texas county east of the Nueces River. Harris County transportation systems serve intrastate and interstate needs with six major railroads hauling freight to distribution centers and to the port; Amtrak provides passenger rail service while METRO provides light rail and passenger bus service. Buses, trucks and passenger cars utilize a network of highways including Interstate 10 east and west and Interstate 45 north and south. U.S. Highway 59 crosses the county from northeast to southwest and goes to the Rio Grande valley, and U.S. 290 leads to West Texas via Austin. Loop 610 encircles the heart of Houston, and a second loop, Beltway 8, allows traffic to move around the perimeter of the urban sector. Two major airports, George Bush Intercontinental and William P. Hobby, are within the city of Houston. Leading sectors include health care, trade, service and education.