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There are trains to carry people, animals or things. There are fast trains and slow ones. Some trains go through hills （穿越山洞）. In big cities, like Beijing or Shanghai, the streets are usually very busy. So you can find trains under the ground （地下）. People call them subways. They carry people quickly in and out of the city. Can a train run over rivers?Yes, it can. There are many bridges over the rivers. But it is not easy to build（建造）the bridges. Today's trains have dining rooms for people. You may eat something in them on the way if you are hungry. Trains are useful to men, women and children. Many children have model trains. With the model trains they can build their own rails （轨道） and enjoy the wonderful trains.