Police said the man stabbed the 25-year-old victim near the Argyle stop in Chicago's Uptown neighborhood at about 1:55 a.m. The attacker left empty-handed and exited the train near the Bryn Mawr stop.
The victim was taken to the hospital and is expected to be okay.
The suspect is described by police as a black man, about 25 years old, wearing a dark knit hat with white letters, a black hooded sweatshirt, black jacket and black jeans. He may have a cut to his face near his right cheek.
Anyone with information is asked to contact the Area north Bureau of Detectives at (312) 744-8263.