The CTA received a report of a disturbance aboard a railcar at the Blue Line Oak Park station about 2:15 a.m., a spokesman for the transit authority said.
Oak Park police were also called for a battery in progress on a westbound Blue Line train.
A witness, who identified himself as Mike, said that disturbance was an attack that left a woman's face bloodied and badly swollen.
There was reportedly commotion on the conductor's radio, then an announcement the train was going to stop at Oak Park, after a man viciously attacked a woman.
"It's absolutely terrible. I don't really know what happened and what really caused that, but I feel very sorry for her," Mike said. "I don't think, I don't think that should ever happen to anybody regardless of the case scenario. And it was just a very terrible scene I kind of wish I didn't see it."
The conductor stopped, and asked all riders to get off the train and call police.
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Oak Park police responded to the scene, and the woman was reportedly taken to Loyola Hospital with serious injuries.
The extent of the woman's injuries was not immediately clear later Thursday morning.
A 26-year-old man was taken into custody on the scene, police said.
The incident took place on the train, but moved onto the Oak Park platform after someone pressed the assistance button near the Austin Boulevard stop, according to police.
A witness told police the incident began with an argument near Kedzie Avenue.
Blue Line trains are currently running normally, the CTA spokesman said.