The problem is it's a short-term benefit, with much of the energy wasted by the time the train stops braking. Now, transit agencies in Philadelphia and other cities across the country are hoping to harness that lost energy by storing it in batteries and putting it back into the system, something that could potentially save millions of dollars in energy costs.
"What we're wasting here is the kinetic energy of a moving train," said Andrew Gillespie, chief engineer of power for Philadelphia's transit agency, the Southeastern Pennsylvania Transportation Authority, who estimates 50 to 70 percent of that power is wasted now.
The idea of storing that power is an enticing one for transit agencies, especially with ridership up and systems expanding. Many of their systems are getting overwhelmed and power costs are spiking.
The Metropolitan Transit Agency in New York is in the early stages of storing such power, while agencies in Los Angeles and Washington, D.C., are looking at doing so, too.
What some electric trains do now is take the resistance created by braking and turn it into energy that goes back into the train car's motor, or onto the third rail where other trains running on the tracks at the same time can use it.
But only about 30 percent of that power is captured, Gillespie estimated, with the rest dissipating, unharnessed.
Working with a private energy company and a $900,000 grant from the Pennsylvania Energy Development Authority, SEPTA is setting out on a pilot program along one of its train lines that would use a massive battery system to store the energy created. Then, that power could be used on the system later or possibly be put back into the grid.
The test run is slated for the Letterly power substation on the Market-Frankford line in Philadelphia's Kensington section.
After it's complete, likely sometime next year, the agency will start testing it and deciding whether to expand the effort to its other 37 electrical substations, Gillespie said.
Ultimately, SEPTA hopes to save up to 10 percent on power costs on a system that takes $22 million a year to power.
The American Public Transit Association is studying the potential benefits of what is known as "wayside storage," storing such energy somewhere other than the rails or the car itself, both of which have limited capacity, said Martin Schroeder, APTA's chief engineer.
"Instead of burning it up on top of the car, you put that additional current into battery storage," he said. The biggest challenge, Schroeder said, will likely be the costs of the batteries, which can be hundreds of thousands of dollars each.
In conjunction with Conshohocken-based Viridity Energy, SEPTA hopes to start construction on its storage devices in 2011.
SEPTA wants to see how it works at one station first. It will gather data by the end of next year and monitor the energy savings to ensure costs aren't somehow subsequently going up elsewhere in the system, Gillespie said.
"SEPTA needs a storage device so they can avoid wasting energy that their trains produce," said Audrey Zibelman, president and CEO of Viridity Energy.
The project could also allow SEPTA to sell surplus energy to PJM Interconnection, which operates the regional grid, Zibelman said. That could potentially generate $500,000 from one storage battery.
"It has a double benefit," she said.
But Schroeder said that might also involve making more and potentially costly changes to the substation.
"There's some trickiness in how the substations work," he said.
That's just one of many things that still need to be studied before transit agencies can know how much braking energy can actually be recycled, and how much money can potentially be saved, he said.