Growling engines, beeping horns, and clattering train wheels are indelible sounds of Chicago's streets. And every day, more than one million travelers hear this din from inside the Chicago Transit Authority's buses and trains. The intricate network of interconnected train tracks and bus routes—which once also included horse cars, trolleys, and streetcars—composes the second-largest public-transportation system in the nation, a system that provides an average of 1.64 million rides per weekday around the city and surrounding suburbs.

Buses zip along 140 routes, pausing periodically at bus shelters or blue-and-white street signs as an automated voice of reason announces each stop in a resonant baritone. The subterranean and elevated "L" train system, whose lines were once named for Saul Bellow characters, switched over to its simpler color-coded system in 1993. Today, for example, the orange line leads to Midway, the blue line takes travelers to O'Hare, the red line connects Wrigley Field and the White Sox's U.S. Cellular Field, and the brown line ambles to the famed Steppenwolf Theatre.

In recent years, the CTA has launched a series of green initiatives, which help conserve energy and cut up to 82 percent of hydrocarbons, carbon monoxide, and other emissions. In addition to offering an alternative to rising gas prices and traffic, the CTA offers LEED-certified headquarters, dispatches fleets of hybrid buses and other efficient vehicles, coordinates recycling programs for engine oil and batteries, and maintains a bike-and-ride system for cyclists who'd like lifts.

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