Final answer:
The third rail in a metro rail system carries a high voltage, typically in the range of 600 to 750 volts DC, to power the trains and operate other systems onboard.
Step-by-step explanation:
The third rail in a metro rail system carries a high voltage to power the trains. The exact voltage can vary depending on the specific metro rail system, but it is typically in the range of 600 to 750 volts DC (direct current).
Powering the trains with high voltage allows for efficient transmission of electricity and minimizes power losses over long distances. The high voltage is necessary to overcome the resistance in the electrical cables and provide enough power to move the trains and operate other systems onboard.