We can find underground railways in many m____ cities.An underground train can start and stop quickly.This helps it to go s___ between stations.The doors can open and close a____.All the systems are e____.Each train can take a lot of passengers.
Unederground trains run as fast as trains above the ground.Of couse,the buses in cities don't run as fast as the underground.Taxis don't,e____.If you are in a hurry,you'd b____ go by underground.It is cheap and c_____.
参考资料:百度知道