(CHICAGO) The CTA’s ambitious $1.7 billion Red and Purple Modernization project will be completed in two phases over four years, starting with the closure of the Lawrence and Berwyn stations for 18 months beginning in 2017.

The CTA announced details of the plan on Wednesday as the agency released its environmental review documents, which are required to apply for federal funding, design a project and gain public feedback, the Chicago Sun-Times is reporting.

The project, announced last spring, will rebuild four stations from Lawrence to Bryn Mawr, as well as a mile of track infrastructure, bridges and viaducts between them on the Red and Purple Lines. The project also involves a bypass or “flyover,” separating the northbound Brown Line from the Red and Purple lines on a new elevated track just north of Belmont. Details on the bypass are pending an environmental assessment, which should be released later this spring.

Here’s how the project will affect commuters if the CTA gets the funding for the hefty project. The transit agency currently has $50 million in state, local and federal funds.

For the first 18 months, crews will close the Berwyn and Lawrence stations, keeping the Thorndale, Bryn Mawr, Argyle and Wilson stations open. During that time, Purple and Red Line trains will run on two tracks, instead of four.

In the second phase of the project — which will last between 18 and 24 months — the CTA will create a new temporary station at Foster and Winona and close the nearby Argyle, Berwyn and Lawrence stations. The Bryn Mawr station also will be open to southbound riders only. Those trains will run on two tracks between the Bryn Mawr and Wilson stations.

Spokeswoman Tammy Chase said the need for repair at these stations is crucial.

“These are stations that are approaching 100 years old,” Chase said. “Right now we are at capacity on certain parts of our rail line. We can’t add more trains. We’re getting to the point where we can’t add additional trains.”

The station redo will allow for elevators. The CTA currently has no wheelchair- accessible stations between the Granville and Addison stations.

Chase said it’s too early to predict any delays riders may encounter during the project.

“Any impact on travel times would be confined to weekday rush service, when Purple Line service is in service,” Chase said.

During the morning rush, about 19,600 rides are taken on the southbound Red and Purple lines from either Howard or Linden to Belmont.

She said it’s most comparable to what riders at the Wilson station are experiencing as crews continue to rebuild a section of track and the station until next spring. That project has added about two to four minutes between Howard and Belmont during the morning rush and between four to six minutes of extra travel time for Purple Line trains between Howard and Belmont during the morning rush.

The CTA initially hoped to acquire three properties for the project, but learned it only needed one property, a used car dealership in the 5600 block of North Broadway. The transit agency has not yet acquired the property. The CTA hopes to use that space to store construction materials.

A public hearing will be held on Thursday, May 14 at 6:30 p.m. at the Broadway Armory, 5917 N. Broadway. The environmental impact study for the Red-Purple Bypass — which has stirred some controversy over its size, and the property it may take to build it — will be released later this spring, with another public hearing being held on the bypass, the CTA said.

Written comments will also be accepted through May 29 via mail.