The train operator who was asleep during the Blue Line crash at O'Hare International Airport also dozed off at the controls in another incident less than two months ago, a federal investigator said Wednesday.

The stunning revelation — that the operator admitted “dozing off” twice — came as National Transportation Safety Board experts wrapped up three days of on-site investigation into how an eight-car subway train could smash through a barrier, leap a station platform and end up atop an escalator.

 But the CTA said it was unaware that the operator was asleep during a Feb. 1 incident and would have taken harsher disciplinary measures against her had it known.

 The agency gave the operator — who is now on injury leave — a written warning after the first incident, saying she claimed to have only “closed her eyes” for a moment, the CTA said.

 The disclosure that the operator nodded off and overshot a station barely two months before Monday's crash has put the CTA on the defensive regarding its disciplinary process and raised questions about what it takes to remove an operator from the job.

 Lead NTSB investigator Ted Turpin confirmed what had been suspected soon after the crash — that the CTA employee, who had only been operating trains 60 days, had been dangerously fatigued.

 “She did admit she dozed off prior to (the train) entering the station. She did not awake again until the train hit,” Turpin said the operator told investigators in an interview.

 The bizarre crash caused an estimated $6 million in damage and forced Blue Line passengers to take shuttle buses from Rosemont to O'Hare. The lead train car was being cut up Wednesday to remove it from the escalator.

 The incident Monday — captured on a video that has gone viral — was the second time the operator acknowledged dozing off at the controls of her train, officials said.

 The Feb. 1 incident was on the Blue Line at the Belmont station, officials said. The operator “dozed off and passed the station without stopping,” Turpin said. “The CTA became aware of that (incident) almost immediately and her supervisor admonished her and had a discussion with her.”

 The train “was a little past the station and she realized it was too late to open the doors,” he said.

 The CTA, however, said Wednesday it was the first they had heard what the operator told the NTSB about the February event.

 The agency said the operator had not told them that she was asleep in February — only that she “closed her eyes for a moment” and overshot a station platform by one car, spokesman Brian Steele said. There is no indication of “dozing off” in CTA records, he said.

 “Had we known she had dozed off, there would have been stronger discipline,” Steele said.

 The February violation was the operator's first safety violation and she was issued a written notice, Steele said.

 Under collective bargaining contract with union, CTA has a “progressive discipline” policy, calling for stronger measures with each violation, depending on the severity. Steele said the CTA expects to initiate disciplinary proceedings against the operator pending the outcome of the NTSB investigation. The process could result in her being fired, he said.

 To discipline or fire an employee, experts say the collective bargaining process generally calls for filing charges against an employee, and a “due process” hearing at which the employee and union get to make their case. If the employee is terminated, the firing can be appealed to a labor board.

 The train operator, who has not been identified, is a member of the Amalgamated Transit Union Local 308. The operator was hired by the CTA in April and sent for training in October. She qualified to operate trains in January, Turpin said.

 The employee worked as a “fill-in” and took on whatever shift was needed at a given time, Turpin said.

 Even if the operator was asleep, questions remain as to why the safety equipment didn't automatically stop the eight-car train. A “trip arm” that activated the train's emergency brakes was 41 feet from the end of the platform, but the train was going about 25 mph, Turpin said.