The stabbing occurred at the Monroe station at about 10:44 p.m. Police said a 49-year-old CTA employee was washing a platform when he accidentally sprayed water on a woman.
The woman became upset and approached the employee before stabbing him in the neck with a knife, police said.
The man was transported to a local hospital in fair condition.
The woman was arrested at the scene and charges are pending, police said.
Area Three detectives investigating.