Greater Sacramento straddles two key regions of California, the Central Valley and the Sierra Nevada mountains and is overlapped by the cultural influences of three areas, the Bay Area, Eastern California and Northern California. An increasing phenomenon taking shape in Greater Sacramento is growth of urban sprawl as Sacramento and its metropolitan area continue to expand. The growth is due in part to first, higher costs of living in the Bay Area which have caused commuters to move as far as Yolo and Sacramento counties and more recently, growth and rising living costs in the core of Sacramento, building up more areas in the surrounding counties for commuters. Sacramento is the largest city in the metropolitan area, home to approximately 500,000 people, making it the sixth-largest city in California and the 35th largest in the United States. It has been the state capital of California since 1851 and has played an important role in the history of California. Sacramento became the largest transportation hub of not only Northern California, but also the West Coast following the completion of the First Transcontinental Railroad. Sacramento today continues to be one of the largest rail hubs in North America, and its rail station is one of the busiest in the United States.