The work to upgrade the signals along the nearly 40-year-old system started Friday at 10 p.m. It's expected to be completed by Oct. 7.
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The nine days of repairs will also include critical track work, requiring the suspension of service to the two stops.
The CTA will be providing free shuttle buses between O'Hare Terminals 1, 2 and 3 and the Rosemont stop.
Riders connecting to the airport's Terminal 5 should use an O'Hare shuttle bus from the other terminals. Buses are set to run every 3-5 minutes during the day, and up to every 10 minutes overnight, the CTA said.
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The CTA urged affected riders to accommodate an extra 20-30 minutes to their travel time.
The repair work is part of the CTA's $207 million improvement project, which transit officials said will eliminate slow zones and outdated stations for the Blue Line service's more than 80,000 daily riders.
The next shutdown will take place sometime next year.