The City of Chicago is undertaking a $492-million, four-year program to overhaul a mass-transit line extending between downtown and Chicago's O'Hare International Airport.

Beginning in mid-2014, the Chicago Transit Authority's (CTA) Blue Line will undergo track and infrastructure improvements, in addition to signal, power and station house upgrades. Rather than complete track replacement, plans call for a series of track improvements along the 12.5-mile line, which consists of subway tunnels, elevated structures and ground-level track along I-90, an expressway linking O'Hare to downtown.

Unlike recent renovations to a line extending between downtown and Chicago's South Side, the Blue Line will remain operational throughout construction, though riders may encounter temporary station shutdowns.

“You can't close the line to the airport,” CTA President Forrest Claypool told reporters Thursday. “O'Hare is the economic engine of the region. “It's critical to commerce, tourism and everything else.”

In addition to tourists and visitors, the line serves 80,000 commuters who use it daily to access Chicago's Loop or points along the city's Northwest side. Ridership on the line has increased 25% in the past five years, according to CTA data.

By eliminating slow zones, CTA officials anticipate that renovations will shave 10 minutes off the 45 minutes currently required to travel between the Loop and O'Hare. Signal and electrical upgrades will allow CTA to increase capacity, while new “turnback tracks” will accommodate shorter runs during peak travel hours.

Chicago Mayor Rahm Emanuel indicated the overhaul is the most comprehensive the line has undergone since it was extended to O'Hare in 1983.