The incident took place on a Blue Line train, the station said, and was posted to Facebook on Wednesday.
CTA said no crime report was filed but the the video was turned over to Chicago police, WFLD said.
Here's the entire clip of the incident via Live Leak. (Content warning: Some rough language ahead):
A WBBM-TV reporter said he spoke with the person who recorded the cellphone video.
“We didn’t do nothing," he told the station's Steve Miller in a phone interview. "We got on the train and he was saying some racial slurs."
The caller identified himself as “A-T-L,” WBBM added, and he said he wasn't part of slapping the passenger.
“I don’t know why they did what they did,” he told WBBM. “I just recorded it because it was funny.”
This story has been updated.