In recent years, a growing number of Chinese cities have begun to build subways in an attempt to reduce traffic jams and air pollution. The subway boasts such merits as safety, high speed and comfort. As a result, an increasing number of people choose subway as the main means of transportation to workplace or school. At this point, it is becoming increasingly convenient to take the subway in China. In some cities, passengers can take the subway just with a card or a smart mobile phone. And many local elderly citizens are able to take the subway for free.