Federal investigators are staying mum about what may have caused the Chicago Transit Authority train to jump its tracks around 3 a.m. Monday, screech across a concrete platform and crash up a heavily used escalator that takes travelers and workers into O'Hare International Airport. Investigators were expected back on the scene Tuesday.
"It is a miracle that nobody died," said Joseph Schwieterman, a transportation expert at DePaul University.
Had the crash occurred during the day, when the trains are often full and the escalator packed with luggage-carrying travelers, far more people likely would have been injured, some even killed, he said. The crash injured more than 30 people, all of whom were on the train, though none suffered life-threatening injuries.
"A train running up a (crowded) escalator could have been a worst case scenario," Schwieterman said. "When pedestrians are hit by a train, it is usual fatal."
He also noted that jumping the track likely dissipated the train's forward movement, thus lessening the accident's severity. A more abrupt stop would have more violently slammed people 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 union representing the train operator said fatigue may have played a role in the crash in a tunnel at O'Hare, the nation's second busiest airport, suggesting the woman may have dozed off.
The operator, who was not immediately identified, had started work at around 8 p.m. on Sunday but had recently put in a lot of overtime, Amalgamated Transit Union Local 308 President Robert Kelly said Monday afternoon.
"I know she works a lot — as a lot of our members do," he said. "They gotta earn a living. ... She was extremely tired.
Kelly said she took standard drug and alcohol tests after the derailment, and he said she assured him they were not an issue.
Asked whether she may have nodded off, Kelly responded, "The indication is there. Yes."
The train is designed so that if operators become incapacitated their 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 onto the escalator.
Investigators had not yet drawn any conclusions about the accident, National Transportation Safety Board official Tim DePaepe said Monday. He said investigators planned to speak to the operator, and scrutinize the train's brakes, track signals and other potential factors.
It's hardly the first time fatigue or a lapse in focus has been raised as a possible contributing cause of a train going off the tracks.
In a December 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 didn't mention that issue, saying excessive speed appeared to be a factor.
In Monday's accident, 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.
Most passengers walked away unaided, officials said. The injured were treated at area hospitals and released.
"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 Monday. "It was a lot of panic."
The train operator suffered a leg injury and has been released from the hospital. Kelly described her 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.
Investigators will also review video footage from a camera in the O'Hare train station and one 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. Meanwhile, the station remained closed, and CTA buses took passengers to and from O'Hare to the next station on the line.
Associated Press videographer Priya Sridhar and writer Lindsey Tanner contributed to this report from Chicago.