Sunday, Mayor Rahm Emanuel stopped by the construction site to greet some of the 1,200 workers involved in the four-year effort.
The initial track work will close small sections of the blue line on weekends only for 10 weeks.
The CTA's latest project, called "Your New Blue" is a $492 million endeavor.
By the end, they promise faster service, modern stations, and more local jobs.
Something Mayor Emanuel highlighted Sunday morning when he visited the site near Milwaukee and Fullerton.
"What's fascinating is all the folks who worked on the Red Line South, that experience, is invaluable to what we are now doing building a New Blue," Emanuel said. "It makes the work all the more efficient, effective, and cost effective for the people of the city of Chicago."
For traveler Dan Scheck who traveled to and from downtown and O'Hare, the shuttle bus service at the interrupted part of the blue line was a breeze even though buses were packed with travelers.
"Got on the bus got off here and we're getting on again," he said.
So why weekends only? Unlike the Red Line project, closing the Blue Line entirely was never an option.
"The Red Line South sits in the median of an expressway and didn't involve street closures, also that portion of the Red Line didn't serve the nation's busiest, or second busiest, airport," said CTA spokesman Brian Steele.
The weekend work will suspend during big summer events like The Taste. After 10 weeks the next phase will begin, which will include improving the stations.
The entire length of the new Blue Line project is expected to last four years.