In recent years, more and more cities in China have begun to build subways, the development of which is conducive to relieve traffic jams and air pollution in urban areas. Due to its advantages of safety, high speed and comfort, a growing number of people choose the subway as their major means of transportation to work or school everyday. Nowadays, it is getting increasingly convenient to take subways in China. Passengers in some cities can simply use cards or mobile phones to take subways and many local elderly citizens can even enjoy free rides.