Briana Hicks is charged with a misdemeanor count of battery, Chicago police said.
A 62-year-old man was on the train in the 300 block of South Dearborn Street at about 11:45 p.m. when police said he accidentally bumped into the woman.
Hicks is accused of taking out a knife and stabbing the man in his thumb, according to police.
The man refused medical treatment.
He's in good condition, officials said.
Hicks is due in court March 19.
Chicago Sun-Times Media Wire contributed to this report.