This was in the Logan Square neighborhood about 6:45 p.m.
Police say a man followed the victim off the train at the Logan Square Blue Line stop, and followed her three blocks, to the 2900-block of North Kedzie Avenue, before knocking her down and sexually assaulting her.
The suspect ran off, and has not yet been found.
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The suspect is described as a 20- to 30-year-old man, wearing a black hooded jacket, dark pants and black sneakers. He was wearing one black glove on his left hand and a purple glove on his right, Chicago police said.
He is about 6-feet tall and 170 pounds, police said.
Anyone with information is asked to call Area Five detectives at 312-746-6554 or submit an anonymous tip at CPDTIP.com.