UPDATED 02/29/12 10:50 a.m.

CHICAGO (CBS) — Police have put out pictures of a man suspected of raping a woman on a CTA Red Line train last week, and the Guardian Angels have been handing out fliers in an effort to find the assailant.

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As WBBM Newsradio’s Bernie Tafoya reports, the 26-year-old woman was sexually assaulted and robbed at knifepoint around 4:55 a.m. Friday, in an otherwise empty Chicago Transit Authority train car somewhere between the Roosevelt Road and 79th Street stops.

LISTEN: WBBM Newsradio’s Bernie Tafoya reports

The woman was riding the train when the suspect, armed with a knife, attacked her. He then got off the train at the 87th Street station, police said.

Police said the woman was alone in the car when she was attacked. It’s unclear how far the car was from the first car, in which the operator might have heard or seen something.

The woman refused medical treatment, police said.

Guardian Angels Chicago Chapter leader Miguel Fuentes says there are ways to play it safe while riding the CTA, especially during times when not so many people are onboard.

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The number one tip, Fuentes says, is that riders should never board an empty car. They should sit in a car with multiple people and preferably the operator too.

Also, Fuentes says: “You want to be alert – not only on the platform, but also when you board the train. You don’t want to board a train that nobody might be on. You might think, ‘Hey! I have a perfect seat!’ You never know who’s going to get on at the next stop.”

One woman had a fatalistic attitude as she headed toward the 87th Street station Wednesday morning.

“You aren’t safe nowhere,” she said. “Nowhere.”

The suspect is black, between 20 and 25 with a slim build, dark complexion and no facial hair. He was wearing a black hooded jacket and all black clothing, and was armed with a black-handled knife with a long blade, the alert said.

CTA spokeswoman Catherine Hosinski said police notified the agency after the assault, and the CTA provided surveillance footage to detectives.

Police released images from the video on Tuesday and are asking anyone with information on the suspect to call detectives at (312) 747-8380 or send tips to AREA1TIP@chicagopolice.org.

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The Sun-Times Media Wire contributed to this report.