A state Department of Transportation spokesman says the system's required under an agreement with the U.S. government.
The GPS-reliant system was in the news recently following an Amtrak derailment in Philadelphia. Safety experts say the system could have helped prevent the crash that killed eight.
Positive train control is already installed on a central Illinois section of the line.
Mike Stead is a manager of an Illinois Commerce Commission safety program. He says once the system's installed, the Chicago-St. Louis line will be among the first major routes with positive control outside the Northeast.