CHICAGO, March 25 - The century-old elevated train system here is as much a city fixture as the towering skyline and the piercing blue waters of Lake Michigan.

But deteriorating tracks and trains, chronic budget shortfalls and a region ever more dependent on rail service are forcing Chicagoans to confront the possibility that the system, commonly known as the El or the L, may be at a breaking point.

"We're living on borrowed time," said Frank Kruesi, the president of the Chicago Transit Authority, which runs the rail service. "The fact is, there's no magic wand when we're looking at modernizing a system that's 100 years old in a very dense urban environment."

The El, with its 1,190 rail cars and 222 miles of track, is the rail component of the transit authority, the second-largest public transit system in the country after New York's. The C.T.A.'s trains and buses serve the city and 40 suburbs, logging 1.55 million rides daily. The El alone accounted for more than 195 million rides last year.