The London Underground (usually called “the tube”) is the biggest subway system in the world. It is also the fastest way to travel in the city. The network consist of nine major routes ( 路线 ), called “lines”. You can buy tickets at any underground stations, at a ticket office, or from a ticket machine. Some of the ticket machines require exact money. All of the machines display “Change given” or “Exact money only” signs. The price of the ticket depends on the length of the trip. The longer the trip is, the higher the cost of the ticket. Trains stop at all stations. On some trains, passengers need to press a button to open doors. Every underground train car has maps that show all the stations on the line you are traveling on. There is a map of the whole underground system in every station. Smoking is not allowed on underground trains or in stations. This passage mainly tells readers how ()