The crime happened Saturday between the Cicero Avenue and Austin Avenue stops in the Austin neighborhood, police said.
Police released a picture of a man who they say performed a lewd act and made sexual comments at a woman before attacking her and trying to sexually assault her.
Chicago police said the suspect is a man who is "40-49 years old, 5'6", medium build, short black hair, full beard and last seen wearing a black jacket unzipped, plain white t-shirt, dark jeans with red and blue bandanas hanging out of the rear pockets and white gym shoes."
The suspect took off at the Austin Avenue stop.
Anyone who recognizes him has been asked to call Chicago police.
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