A lot of energy is wasted each time a train stops at one of the 28 stations on SEPTA's Market-Frankford Line.

In a six-car train, the brakes produce about 3 million watts of power during the 15 seconds it takes to halt the 400 tons of hurtling metal, plastic and humanity. Some of the electricity is reused by other trains on the line. Much of the power is lost — dissipated as hot air through the subway car's rooftop vents.

But what if the electricity produced by the train's regenerative brakes could be captured and reused, as it is with a hybrid vehicle? And what if the power could be resold at a profit?