Philadelphia's Subway Trains Are Becoming Rolling Power
Factories  The transportation sector accounts for nearly half of
United State's total energy consumption. With fuel prices
continually climbing, those energy costs are cutting into
already-strained state budgets. To combat its carbon footprint,
the South East Pennsylvania Transit Authority (SEPTA) has
outfitted its light-rail trains with regenerative braking
technology. That's right: They'll power the system every time
they slow down.  SEPTA's pilot program along the Market-
Frankford and Broad Street lines is designed to absorb the
energy generated by slowing trains in five downtown stations and
feed it into a 800-kilowatt, 400-kilowatt-hour battery at a
nearby substation. When a SEPTA train slows as it approaches a
station, it uses an electric motor to convert kinetic energy to
electrical, and then dumps the excess charge into the overhead
third rail. A six-car train can generate as much as three
megawatts in 15 seconds of braking. The problem, though,