It happened around 5:25 p.m. back on October 31 when the 38-year-old victim was on the EL train at Columbus Boulevard and Spring Garden Street.
Police say the suspect punched the man several times in the face and head, which caused a concussion.
On Tuesday, police released images of the suspected attacker.
Further details on the incident have not been released.
Anyone with information is asked to call police at 215-686-TIPS.