CPD said the incident took place just after 3 p.m. March 21 in the 900-block of West Belmont Avenue in Lakeview.
Police said the suspect committed aggravated battery against an elderly person on the platform.
The suspect is described as wearing a yellow hooded sweater, black jeans and white sneakers.
Just days later, an 82-year-old man was beaten at a South Loop CTA station.
CPD did not immediately provide any additional information on what led up to the attacks. It's not clear if they're related.
Anyone with information is asked to call Public Transportation Detectives at (312) 745-4447 or submit an anonymous tip at CPDTIP.com.
INTERACTIVE SAFETY TRACKER | Track crime and safety in your neighborhood