Dallas is the third largest city in Texas and the largest city in the Dallas–Fort Worth metroplex, the fourth-largest metropolitan area in the United States at 7.5 million people. It is the largest city in and seat of Dallas County with portions extending into Collin, Denton, Kaufman and Rockwall counties. With a 2020 census population of 1,304,379, it is the ninth most-populous city in the U.S. and the third-largest in Texas after Houston and San Antonio. Located in the North Texas region, the city of Dallas is the main core of the largest metropolitan area in the southern United States and the largest inland metropolitan area in the U.S. that lacks any navigable link to the sea. The cities of Dallas and nearby Fort Worth were initially developed due to the construction of major railroad lines through the area allowing access to cotton, cattle and later oil in North and East Texas.