The robbery took place at about 3:46 p.m. on December 12 at the Roosevelt station, police said.
Police said the victim was on the platform when the man approached, indicated he had a gun and took his cell phones.
The suspect fled the area. He is described by police as 18-25, 5'05"-5'09" and 150-180 pounds.
Anyone with information is asked to contact Area Central Detectives at (312) 747-8380.