Today, many of the trains do go underground, but it's best known for its elevated trains, featured in movies like "The Blues Brothers" and "Risky Business."

The first Chicago trains were operated by the Chicago South Side Rapid Transit Railroad and began service in 1892. The Green Line still uses some of the track, but (thankfully!) the trains have been replaced from the wooden coaches that carried a dozen people each.

The Chicago Transit Authority took over in 1947 and modernized service into the building blocks of what operates today, though trains and routes are continually being upgraded to reflect ridership needs. Today, the L runs over a total of 224.1 miles of track and boasts an average of almost 600,000 riders every weekday.

More Info: en.wikipedia.org