CHICAGO — The CTA board just approved contracts for the largest and most expensive project in its history.

The Red and Purple Line modernization is expected to cost $2.1 billion.

That modernization project will include a Brown Line flyover north of the Belmont station, so Red and Purple trains don’t have to sit and wait there.

In the end, the CTA says this will bring better rush hour service with less congestion.

The work between Lawrence and Bryn Mawr is being paid for with federal funds and tax increment financing.

Ten buildings in the Lakeview neighborhood have already been demolished with this project in mind.

The reconstruction begins in the Fall of 2019.

The Red, Brown, and Purple Lines will stay up and running, with some interruptions, until everything is done in 2025.

An even more expensive project is also in the works.

The CTA board is approving preliminary work on extending the Red Line from 95th Street to 130th Street.

That project proposal adds up to $2.3 billion.

That preliminary work is for contracts on environmental impact studies, which can lead to more federal funding.

The CTA is also looking for more state capital improvement funds.

The Red Line is the busiest line in the system, giving 74 million rides a year.