If subway stations could harvest the energy of incoming trains as they hit their brakes, they could re-use that same energy to launch the same train as it leaves the station. That's the idea behind Vycon flywheels, which are gigantic spinning wheels that act as temporary energy storage mechanisms.

The energy savings of having a flywheel in a station could be tremendous, says the company. In 30 seconds, a decelerating subway train with electric brakes can put up to 4 megawatts of power into a flywheel via the station’s electric third rail — that's enough to run 1,300 average U.S. homes.

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The company also claims that up to 90 percent of that energy can be recovered in order to launch the train from the station once passengers have boarded. Multiply that process across the thousands of stations in the New York City subway system, and it could be a mind-boggling amount of energy saved.

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