The Monday, July 9, 2012, incident happened around 9:29 p.m. on a CTA train approaching the Belmont "L" stop, said police.
Authorities did not say if the sex offense occurred on the Red, Purple, or Brown Line.
The offender was described as a black male in his 20s, about 6-feet-tall, wearing a blue, plaid shirt.
The suspect got off the train at Belmont and was last seen the Belmont and Halsted Street area.
According to a post at an online community forum, another woman, who apparently lives in Lakeview, experienced a similar incident on a southbound Red Line train on July 6, 2012, around 10:30 p.m.
The woman said a male passenger purposely exposed his genitals while popping chewing gum and staring at her.
"I was heading south into downtown on the redline this past Friday when I noticed the privates of the man sitting across from me were sticking far out of his shorts. He was staring directly at me, popping his gum.
I refuse to believe there was some way he wasn't aware of what he was doing - he kept shifting I think to grab my attention,
This was a more than half full train, about 10:30 pm. I was by myself, frightened and disgusted, and after about 2 minutes got enough guts to move to the other side of the car. Two days later I'm wishing I would've called attention to him or screamed or something," said the frightened woman.
With the attempted rape on Waveland Avenue still unsolved, these reports are extremely unsettling.