Chicago police said a man fell onto the tracks at the Clark/Lake station in the Loop at about 1:59 a.m.
He died at the scene. Police are conducting a death investigation.
Video shows as the station in the Loop was closed for hours.
There was no train service between Grand and UIC-Halsted. The CTA said shuttle buses were available from Grand to UIC-Halsted.
All services were restored by 6:30 a.m. CTA said some trains and platforms may be busier than usual.