The Chicago Transit Authority is a step closer to the biggest and most expensive construction project in its history.

The Chicago Tribune reported that on Wednesday, the CTA board approved contracts for a $2.1 billion modernization project for the Red and Purple Lines that will include the rebuilding of stations, bridges and track.

The project will include construction of a bypass that will take Brown Line trains above the Red and Purple Line trains north of the Belmont station on the city's North Side.

Construction is expected to begin next fall and the CTA anticipates it will be completed in 2025.

At the same time, the board approved a contract for a preliminary work on the proposed $2.3 billion extension of the Red Line on the city's far South Side.