There are trains to carry people，trains to carry animals or carry things. There are fast trains and slow ones. Some trains go through hills. In big cities，the streets are usually very busy. So you can find trains under the ground. they carry people quickly in and out of the big cities. Can a train run over the water? Yes，it can. There are many bridges over the rivers. The trains can easily run over the water. But it is not easy to build 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(铁轨) and enjoy the wonderful world of trains.