We can find underground railways in many modern（现代的城市都有地铁） cities.An underground train can start and stop quickly.This helps it to go straight（笔直地） between stations.The doors can open and close automatically（自动开关车门）.All the systems are efficient(效率高的).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,either(“也”没有地铁快速).If you are in a hurry,you'd better（最好） go by underground.It is cheap and convenient（便利的）.