CHICAGO (CBS) — Police are looking for a suspect, after a woman’s simple commute on the CTA Green Line was shattered by a robber with a semi-automatic weapon.

Police say the robber, whose image was captured in surveillance photos, held up a 20-year-old woman at gunpoint on the Green Line platform at 43rd Street.

The woman said the robber was armed with a black semi-automatic pistol. He was described as being about 50 years old and having a pony tail.

Police advise anyone taking the ‘L’ to be aware of their surroundings, remain calm if confronted by an assailant, remember unique physical characteristics, never pursue a fleeing assailant, request contact information if approached by a witness, dial 911 immediately, and remain on the scene if possible.

Anyone with information is asked to call the Area Central Bureau of Detectives, at (312) 747-8380.