These years you can see many underground railways are being built in some big cities. underground railway is both convenient and fast. the underground railway is also called "the tube","the subway'or "the underground".
　　There're lifts for you to go up or down. the trains' doors open and close automatically. in shanghai, during rush hours the underground arrives at the station every two minutes.
　　But with too many people taking the underground, people find it really too crowded. maybe shanghai needs more underground railways.