The driver of the Chicago Blue Line train that derailed, injuring dozens of people, has told investigators that she fell asleep at the controls.

"She did admit that she dozed off prior to entering the station and did not awake again until the train hit close to the end of the bumper," said NTSB lead investigator Ted Turpin.

"She woke up when she hit," he continued. "She didn't really remember, but she made an assumption she must have or the train would have stopped."

Turpin said that the driver was "very cooperative and very forthcoming" about the derailment of the train at Chicago's O'Hare Station.

Turpin said that officials were to investigate the driver's work cycle and find what she does in her spare time to see what contributed to her tiredness.

The derailment cost $6m after the train careered past the end of the track and on to the escalator at O'Hare International Airport. Thirty-two people were injured but none of the injuries were life-threatening. The train was travelling at 26mph when it derailed.

"The train actually climbed over the last stop, jumped up on the sidewalk and then went up the stairs and escalators," said Chicago Fire Department commissioner Jose Santiago.

Officials have finished their onsite investigation and will allow the Chicago Transit Authority to remove the train.