The girl was riding the train on January 8 around 4:40 p.m. when a man standing behind her began rubbing her buttocks, according to police.
She told the offender to stop. He stopped and sat down, authorities said.
The victim left the train at 35th and Archer and told police about the incident, police said.
Police tried to catch the alleged offender at the next stop but did not apprehend him, they said.
The alleged offender is described as a white male who has white hair, around 55-60 years old and 5'07" to 5'08", according to authorities. He was wearing a tan jacket with black shoes, they said.
Authorities recommend people stay alert and aware of their surroundings.