Among the heaviest of mass transportation vehicles, subway trains require high levels of electrical power for acceleration and general movement on steel rail track networks. The constant acceleration and deceleration, starting and stopping of subway trains deprives them of any of the economies of steady, long range freight trains which run on essentially the same type of steel rails and steel wheels.
The fundamental nature and design of subway trains leads to very limited opportunities or options for reducing demand and consumption of electrical power. Inextricably dependent on high voltage electrical power for their operation, subway systems are vitally connected to the electrical distribution grid of their local area and region, and are subject to the cost and reliability characteristics of the electrical power distribution network. Power failures on subways usually create significant inconveniences, if not emergencies, requiring response and services of emergency responders in the case of extended disruption of subway service and equipment.