Situated in northern California on the Unites States' west coast, San Francisco is populated by 6.3 million people in its city and surrounding areas. Known for its hilly landscape, its Bay and famous bridges, San Francisco city is made up of a mass of borough-style areas, most of which are densely populated. The transport serving these areas is extensive, and is provided by a variety of transit modes. The BART (Bay Area Rapid Transit) is the main provider of suburban heavy rail services thoughout the San Francisco, Fremont, Berkeley and Oakland areas, whilst Amtrak and Caltrain operate commuter services to outerlying areas in California and interstate. MUNI (San Francisco Municipal Railway) operates a vast network of buses, trolley buses, trams and light rail thoughout the San Francisco area, and a significant number of other bus operators run services to other Bay destinations.