Chicago Fire Commissioner Jose Santiago says the train "climbed over the last stop, jumped up on the sidewalk and then went up the stairs and escalator."
Chicago Transit Authority spokesman Brian Steele says authorities are trying to piece together what happened, but says the train was likely traveling too fast.
The derailment happened around 2:50 a.m.
Investigators aren't sure how many people were on board, but say none of the injuries are believed to be life-threatening. The injured were taken to four hospitals.
The CTA is busing passengers to and from O'Hare to the next station.
PHOTO CREDIT: NBC