THERE'S NO END TO THE SERVICE THEY PROVIDE MANKIND:

Subway Trains to Generate Power for the Grid (Kevin Bullis, September 10, 2010, Technology Review)

The Southeastern Pennsylvania Transportation Authority (SEPTA), which runs the transit system in Philadelphia, is piloting a smart electrical grid technology that could cut its electricity bills by up to 40 percent and generate millions of dollars a year.
Powering down: A new project will harness energy from subway trains like this one on Philadelphia’s Market-Frankford Line.
Credit: SEPTA

A massive battery installed at one of the authority's substations will store electricity generated by the braking systems on trains (as the trains slow down the wheels drive generators). The battery will help trains accelerate, cutting power consumption, and will also provide extra power that can be sold back to the regional power grid. The pilot project, which involves one of 38 substations in the transit system, is expected to bring in $500,000 a year. This figure would multiply if the batteries are installed at other substations.

The project shows how cash-strapped public transit agencies that operate major subways and electric rail systems could find a new source of income by tapping into the smart grid. It also highlights one way that the smart grid could save energy, avoid blackouts, and incorporate more renewable power.