The driver of a Chicago commuter train that crashed at O'Hare International Airport admitted she "dozed off" before the accident, waking only when the train jumped off the tracks and climbed an escalator, a federal official says.
National Transportation Safety Board (NTSB) investigator Ted Turpin says the woman had been working as an operator for about two months and acknowledged she'd previously fallen asleep on the job in February, when her train partially missed a station.
"She did admit that she dozed off prior to entering the station," he said during a briefing on Wednesday.
"She did not awake until the train hit."
He said the woman, who was co-operating with the investigation, often worked an erratic schedule, filling in for other Chicago Transit Authority employees.
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"Her hours would vary every day," he said.
Turpin said the NTSB is investigating the woman's training, scheduling and disciplinary history.
"The CTA became aware of that (February incident) almost immediately and a supervisor admonished her and had a discussion with her," he said.
More than 30 people were hurt during the crash, which occurred around 3am, but none of the injuries were serious.
Turpin said the crash caused about $US6 million ($A6.56 million) worth of damage.
Meanwhile on Wednesday, the station remained closed as crews cut apart the lead car of the eight-car train that was damaged during the crash.
