Departing trains at a rail station could someday get their initial oomph for free, each time saving the equivalent of several days' worth of electricity usage by an average U.S. household. The trains would rely on a concept already used in today's hybrid gas-electric cars: reuse of energy stored while braking to a stop. But while hybrid cars stockpile energy in massive arrays of batteries, the heart of the hybrid train might be a deceptively low-tech device—a flywheel.
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