Police released surveillance photos of the man Tuesday, but said the incident took place on June 24.
Police said at approximately 9 a.m. a victim was on a Brown Line train near the State and Lake Stop when she was approached by the man, who demanded money and then pushed the victim onto the seat.
Police said the suspect then tried to get into the victim's pockets and undo her pants, but she was able to fight him off and make her escape.
If you have any information about the man in the photos, contact Chicago Police Bureau of Detectives for Area Central at 312-745-4706.