On the heels of a massive reconstruction project on the CTA's Red Line, the Blue Line will soon be upgraded.

The city unveiled plans Thursday for the largest modernization of the CTA Blue Line since the O’Hare branch was built 30 years ago, a four-year overhaul that officials say will shave 10 minutes off travel between downtown and the nation’s second-busiest airport.

But things are going to get slower before they get faster.

The $492 million track and station rehabilitation project beginning in the middle of next year will mean rolling delays for the more than 80,000 weekday commuters who board at stations on the branch as well as visitors who ride the train into the city after landing at O’Hare, transit officials said.

The CTA will try to minimize the hassle by offering travel alternatives such as shuttle buses and will schedule shutdown-causing track work for overnights and on weekends. But what is key to the upcoming project, and what sets it apart from the recently completed Red Line reconstruction, is that the Blue Line will stay open.

“There will be some temporary closures and impacts to customers,’’ said Christopher Bushell, CTA chief infrastructure officer. “We need to get a little further into the schedule development. But there won’t be any major changes to service.”

Unlike the recently completed $425 million reconstruction of the 10-mile stretch of the Red Line Dan Ryan branch, which was completely shut down between mid-May and mid-October, the 12.5-mile overhaul of the Blue Line O’Hare branch represents much different priorities and challenges that will result in a longer-term pain for riders until they reap the gains, officials said.

By eliminating slow zones and allowing trains to operate at top speeds of 55 mph on longer stretches, CTA officials predict that travel times between downtown and O’Hare, which now take at least 45 minutes, will be cut by up to 10 minutes each way.

It’s the same time savings that Red Line South branch riders now enjoy between downtown and the 95th Street terminal.

But that’s where the similarities between the two projects end.

The Red Line South project was mostly at ground level along the median of the Dan Ryan Expressway. And the branch does not serve a major airport. Work during the five-month line shutdown involved building essentially a new railroad from the sub-base to the tracks and signal system, along with improvements at eight stations. Construction will begin in the spring on a $240 million project to rehab and expand the 95th Street terminal.

The O’Hare branch of the Blue Line includes elevated structure, subway tunnels and ground-level track on the median of the Kennedy Expressway. The upcoming project, called “Your New Blue,’’ is not a complete track replacement, as was done on the Red Line South. Instead, the work involves a series of track and station improvements between the Grand and Cumberland stations.

CTA officials said they never considered a complete shutdown of the O’Hare branch, which according to the CT has seen a 25 percent ridership increase in the last five years.

“It’s really a completely different kind of project,’’ Bushell said. “It requires a different approach in terms of closures (because) it’s a different kind of program that appeals to a broader base.’’

The signal system will also be upgraded between the Jefferson Park and O’Hare stations to help increase train speeds, and an improved electrical power supply will provide increased capacity for the CTA to run more trains, as well as to improve service reliability, officials said.

“Turnback tracks’’ will also be constructed at Jefferson Park and UIC Halsted to give CTA rail dispatchers the option to short-turn trains to pinpoint service to meet the heaviest demand during peak travel hours, officials said.

The project schedule has not been finalized, officials said. But it is expected to begin with track work, followed by renovations at 13 stations, power upgrades and signal improvements, Bushell said.

Wireless infrastructure will also be upgraded to support 4G service for passengers using mobile phones and other electronic devices in the Dearborn Street and Milwaukee Avenue subways, officials said.

In addition, renovations, similar to the facelifts conducted in 2012 and early this year at seven CTA stations on the Red Line north branch, will be done on the Blue Line stations at Grand, Chicago, Division, Damen, California, Logan Square and Jefferson Park, the CTA said.

Less extensive repairs will also be carried out at the Irving Park, Montrose, Harlem and Cumberland stations, officials said.

An elevator will be installed at the Addison station to make it accessible to individuals using wheelchairs.