Chicago Fire Commissioner Jose Santiago said the train “climbed over the last stop, jumped up on the sidewalk and then went up the stairs and escalator”.
Initial reports said none of the injuries were believed to be life-threatening.
The Chicago Transit Authority is trying to piece together what led to the accident, but a spokesman said it appeared the train was travelling too fast.
“Apparently (it) was travelling at a rate of speed that clearly was higher than a normal train would be,” said the spokesman, Brian Steele.
The derailment happened around 2.50am local time (7.50am UK time).
The train was being removed and the escalator fixed, Mr Steele added.
Article Appeared @http://news.sky.com/story/1230976/chicago-airport-train-derailment-injures-32