The early phase of construction will not affect service at the current station.
But lanes are closed on Lake Street.
Lake from Dearborn Street to Wabash Avenue has been reduced to one lane.
This week, State Street lanes will be shut down.
State Street from Lake to Randolph Street will be reduced to one lane in both directions.
The initial work is expected to last three to four months.
Artist renditions of what the new station will look like show a larger facility with a glass ceiling.
The project replaces the 100-year-old station with a new station with wider platforms and new elevators.
Station construction is expected to be completed in 2029.