The project would be the biggest investment in the 19.5-mile Blue Line since the line was extended from Jefferson Park to O'Hare International Airport in September 1984, city officials said. Work would include track and infrastructure improvements, station house upgrades, subway right-of-way improvements, and signal and power upgrades.
CTA officials hope such improvements will reduce the 50-minute travel time between O'Hare and downtown Chicago by up to 10 minutes.
The plan follows a similar $425 million effort directed at the CTA Red Line, which like the Blue Line operates 24 hours a day.
The Blue Line, spanning roughly 35 miles and serving 33 stations, runs roughly southeast from the airport through downtown Chicago (underground when it traverses the Loop), turning toward the southwest to terminate at Forest Park.