CHICAGO (AP) -- An operator of a Chicago public-transit train that jumped the tracks and scaled an escalator at one of nation's busiest airports Monday may have dozed off, the president of a Chicago transit union said.
The woman said she had worked extensive overtime recently and was "extremely tired" at the time of the accident, Amalgamated Transit Union Local 308 President Robert Kelly told a news conference.
The derailment happened just before 3 a.m. Monday at the end of the Chicago Transit Authority's Blue Line at O'Hare International Airport. The timing of the accident helped avoid an enormous disaster, as the underground Blue Line station is usually packed with travelers. More than 30 people were hurt, but none had life-threatening injuries.
A CTA supervisor and another worker near the top of the escalator said they saw the train enter at a normal rate of speed, about 15 mph, according to Kelly.
"The next thing they heard the sound (of impact) and the yelling and the screaming," he said.
Investigators had not drawn any conclusions about the cause of the accident, National Transportation Safety Board official Tim DePaepe said Monday afternoon, but that were looking into whether faulty brakes, signals or human error were factors.
The train is designed so that if an operator becomes incapacitated and his or her hand slips off the controls, it should come to a stop. Kelly speculated that, upon impact, inertia may have thrown the operator against the hand switch, accelerating it enough to send it catapulting onto the escalator.
"I heard a `Boom!' and when I got off the train, the train was all the way up the escalator," passenger Denise Adams told reporters. "It was a lot of panic."
The train operator, who has worked for the CTA for about a year, suffered a leg injury and has been released from the hospital. She will be interviewed by investigators Tuesday, Kelly said.
Asked by a reporter whether she may have nodded off, Kelly responded, "The indication is there. Yes."
Kelly described the train operator after the accident as distraught, but still able to help passengers.
"She immediately got out of the cab and started asking everybody and checking to make sure that everybody was OK," he said.
Jumping the track likely dissipated the forward movement, thus lessening the accident's severity, said Joseph Schwieterman, a transportation expert at DePaul University.
A more abrupt stop would have slammed people more violently into the train's seats and walls, he said.
"That was a lucky break," he said. "A train hitting a wall at ... high speed could easily have been fatal for many."
The injured were treated at area hospitals and released. Chicago Fire Commissioner Jose Santiago said Monday morning that most were able to walk away from the wreck unaided.
Investigators will review video footage from a camera in the O'Hare station and one that was mounted on the front of the train, DePaepe said. The train will remain at the scene until the NTSB has finished some of its investigation, after which crews will remove the train and fix the damaged escalator.
CTA spokesman Brian Steele said earlier Monday the train may have been moving too fast as it approached the station and didn't stop at a bumping post -- a metal shock absorber at the end of the tracks.
Fatigue or temporary inattention have been raised as possible factors in other commuter train accidents.
In December's train derailment that killed four people in New York, representatives of the operating engineer have said he may have lost focus at the controls in a momentary daze. A preliminary report did not mention that issue, saying excessive speed appeared to be a factor.
That accident highlighted the lack of crash-avoidance systems, or "positive train control," which uses GPS, wireless radio and computers to monitor a train's position. It lets dispatchers halt engines remotely if they speed or blow through stop signals.
It's not clear such a pricey system could have helped prevent Monday's derailment in Chicago.
"There are systems that do stop trains," DePaepe said. "But it is usually about money. The transit agencies do the best they can."
Monday's accident occurred almost six months after an unoccupied Blue Line train rumbled down a track for nearly a mile and struck another train head-on at the other end of the line in September. Dozens were hurt in that incident, which prompted the CTA to make several safety changes.
While the station is closed, the CTA will bus passengers to and from O'Hare to the next station on the line.