There have been several robberies within the past week.
RELATED: River North shooting: Teen killed by gunman waiting outside Grand and State Red Line stop, CPD says
The CTA said guards will have a more visible presence on the system during key hours of operation.
Friday morning, Chicago police released photos of three people suspect of attacking and robbing a man on the Red Line.
RELATED: Chicago police detectives investigating rise in CTA crimes after shootings, violence along Red Line
It happened Monday at the Grand Station in the Near North Side neighborhood.