Rapid Transit, also known around the world as subways, metros, tubes, rapid transit, and the underground, are a form of high-capacity public transportation that operate as separate below-grade or elevated railways in cities. Made up of multiple simultaneous lines of railway tracks and designated station locations, rapid transit systems are an essential piece of infrastructure in our ever growing urban centers. Rapid transit systems today are the primary form of transportation in cities with the largest ridership in Tokyo, Seoul, Moscow, Beijing, Shanghai, Guangzhou, New York City, Mexico City, Paris, and Hong Kong.