In recent years, more and more cities in China have begun to build subways. The development of subways can help reduce traffic congestion and air pollution in cities. The subway has the advantages of safety, speed and comfort. More and more people choose the subway as the main means of transportation to work or school every day. Nowadays, it is becoming more and more convenient to take the subway in China. In some cities, passengers can use a card or a mobile phone to take the subway. Many local elderly citizens can also take the subway for free.