National Transportation Safety Board investigator Ted Turpin said 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 today. "She did not awake until the train hit."
"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.
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More than 30 people were hurt during the crash, which occurred around 3 am (local time), but none of the injuries were serious.
Turpin said the crash caused about USD 6 million worth of damage.