Brown Line service has resumed after the incident in the 1800-block of West Addison Street.
Trains were suspended between Belmont and Kimball on the North and Northwest sides for a couple of hours.
Chicago fire officials said one person was taken to a nearby hospital in critical condition and another in good condition after the incident just before 10 a.m.
The Cook County Medical Examiner's Office later said it is conducting a death investigation.
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