They released three different pictures. They do not know the man's identity but hope to talk with him as soon as possible and are asking for the public's help.
A man attacked the woman near the Morse Red Line station, dragged her to an area where there is ongoing construction and that is where she was sexually assaulted.
Police say that the man in the photos they released on Wednesday night was riding the CTA's Red Line between 11 p.m. Tuesday and 5:30 a.m. Wednesday.
He is 6-feet tall and weighs about 200 pounds.
"We believe that this individual was on the L, was on the Red Line at some point for a period of time in the early morning hours. At some point then arriving at the Morse L station, which is when he then approached the victim and the attack was complete" said Chicago Police Sargent Mia Ogliore.
Chicago Police say that the woman is in the hospital in stable condition.