Science: The Big Wheel

To passengers on San Francisco's "Muni" (Municipal Railway) system, the two new vehicles will look—and ride—like ordinary electric trolley-buses. But when they begin rolling up and down the city's hills next year, transportation engineers everywhere will be eagerly watching their progress. The test vehicles will be pushed along part of their routes by a spinning flywheel.

Among the most ancient of mechanical devices, the flywheel works on a simple principle: a rapidly spinning weighted wheel serves as a highly useful reservoir of energy. It has been put to work in a wide variety...