The crime happened Friday morning on a CTA Yellow Line train in Evanston, police said.
After an attack happened on a train, a male suspect fled the scene and a victim was taken to a hospital for injuries, police said.
The victim was later released from the hospital. Their age and gender were not released.
Evanston police released images of the suspect Saturday and were later able to locate him at the Howard Street CTA Station in Chicago with help from public tips.
The suspect is in custody and charges are pending, police said. His age and identity were not yet released.
No further information was available.