Subway Construction

　　Tunnels: Many tunnels run by underground railways are smaller than those on main trunk lines; therefore, in general, the volume of trains on underground railways is generally smaller. Sometimes the tunnel can even affect the shape design of the train, such as some trains on the London Underground.

　　Power: Most urban rail systems use distributed power (ie, EMU trains) instead of centralized power. If you use a power-centralized type, you will often use push-pull operation.

　　System: Some of the more advanced systems have begun to introduce automatic train operating systems. The captains of London, Paris, Singapore, Guangzhou, Taiwan and Hong Kong do not need to control the train. More advanced rail transit systems can be unmanned. For example, the world's longest automated LRT (light rapid transit system) system—Vancouver Skytrain, all stations and trains in the entire LRT are "unmanned." Both Guangzhou Metro APM Line and Shanghai Rail Transit Line 10 are driverless, and drivers only monitor.

　　Vehicles: The original urban rail system vehicles were made of wood, but they were later changed to steel to reduce the risk of fire. Since the Toronto Underground Railway opened in 1953, the vehicles have been improved to aluminum, effectively reducing maintenance costs and weight.