Power Supply System of Metro Sam: Good morning. Lily: Good morning, sir. What can I do for you? Sam: I feel so puzzled about the power supply system in the subway. Could you introduce it for me? Lily: It mainly consists of city electricity, LV distributor, UPS, computer controlled smart UPS and subsystems. There are two power supplies for the signal system of metro. Sam:I see. There are two power supplies for the signal system of metro. Lily:Yes, you are right. In addition, there are two modes for providing the power to the metro, i e. Third rail and overhead wires. Sam: What is a third rail? Lily: A third rail provides electric power through additional rail (called a “conductor rail”) placed alongside or between the rails.