The Mexico City subway, or metro, is one of the most extensive and modern subway systems in North America. The metro features several lines running below Mexico City and serving outlying suburban communities. Most of the Mexico City subway cars run on rubber rather than metal wheels.
Heavy-rail systems are also commonly referred to as rail rapid transit. Subways (often called metros outside of the United States) are common examples, although rail rapid-transit systems may also operate above ground, as parts of the New York City and Chicago, Illinois, subway systems do. Heavy-rail systems typically consist of large four-axle rail vehicles operating in trains of two to ten cars. Rail rapid-transit systems operate on tracks reserved solely for the rail cars, and so the trains are able to travel at high speeds. Some rail rapid-transit systems, such as BART (Bay Area Rapid Transit) in the San Francisco Bay area, are highly automated. Power for rail rapid-transit vehicles is usually supplied by an electrified third rail mounted alongside the train tracks. Some heavy-rail systems use rubber tires rather than steel wheels. These tires produce a quieter ride but create more friction, which reduces efficiency. A monorail is a special type of heavy-rail system that uses a single rail to support and guide the vehicles. A monorail that was built for the 1962 World’s Fair in Seattle, Washington, connects the downtown to the nearby fairgrounds and is still in use. Monorails have also been built for circular routes around airports or at amusement parks, such as Walt Disney World, but they have not been widely used for urban transportation.