EPTA’s Market Frankford line, the busiest transit line in Philadelphia and referred to locally as the “El,” has moved commuters, shoppers and visitors for more than 100 years. Moving these trains requires a large amount of electricity. In 2010, SEPTA partnered with Viridity Energy to implement a new strategy to cost-effectively reduce SEPTA’s energy consumption. The El’s electrically powered trains also have regenerative braking capabilities. When trains brake, their electric motors produce energy—a six-car train on the Market-Frankford Line can produce up to three megawatts in 15 seconds of braking. The problem is, unlike hybrid vehicles, 20th century trains were not designed with an energy storage capacity. Regenerated energy from a braking train can only be used if there is another train accelerating in the immediate area. Otherwise, the regenerated energy is wasted—dissipating as heat through resistor banks on the top of the train.