A US transport union official says there are indications the operator of a commuter train that derailed at Chicago's O'Hare International Airport had fallen asleep before the crash.
Amalgamated Transit Union Local 308 President Robert Kelly said the operator told him she had worked a lot of overtime recently and was "extremely tired."
The eight-car public transit train jumped the tracks, skidded across a platform and scaled an escalator leading to the airport. More than 30 people were injured.
The National Transportation Safety Board planned to interview the train operator, who was still hospitalised.
Investigators were also looking into whether faulty brakes, signals or human error were factors, investigator Tim DePaepe said.
The timing of the crash - just before 3am on Monday (local time) - helped avoid an enormous disaster, as the underground Blue Line station is usually packed with travellers coming to and from Chicago. No one suffered life-threatening injuries.
A four-year, US$429 million (NZ$502m) renovation recently has begun on the Blue Line from O'Hare to downtown.
The accident occurred almost six months after an unoccupied Blue Line train rumbled down a track for nearly 1.6 kilometres and struck another train head-on at the other end of the line.
Denise Adams, a passenger on the train, described the impact to the Chicago Sun-Times.
"I heard a 'Boom!' and when I got off the train, the train was all the way up the escalator," she said. "It was a lot of panic."
Investigators will review video footage from a camera in the 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 investigators, after which crews will remove the train and fix the escalator that has "significant damage."
Hours after the crash, the front of the first car could still be seen near the top of the escalator.
While the station is shut down, the CTA was busing passengers to and from O'Hare to the next station on the line.
The train appeared to have been going 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.
"Apparently (it) was travelling at a rate of speed that clearly was higher than a normal train would be," CTA spokesman Brian Steele said. He also said it wasn't clear how many people were on board at the time of the crash, but that it took place during what is "typically among our lowest ridership time," Steele said.
The injured were taken to area hospitals and Chicago Fire Commissioner Jose Santiago said that most were able to walk away from the wreck unaided.
Evonne Woloshyn, a spokeswoman at Advocate Lutheran General Hospital in Park Ridge, said seven people were treated for minor "whiplash-type" injuries after complaining of head and neck pain.
Chicago's 386-kilometre subway system, which had fallen into disrepair in recent decades, is known locally as the L and been an instantly recognisable backdrop in movies such as "The Blues Brothers."
Other lines have been renovated in recent years, including an overhaul on the south end of the Red Line that wrapped up in October.
The Blue Line, which still has stations built in the late 1800s, was extended to O'Hare in the early 1980s. The current project is the biggest investment since the extension and will upgrade a 20-kilometre stretch in order to speed up travel times between O'Hare and the city's downtown.
The September accident, which happened on the other end of the Blue Line, hurt dozens of people and prompted the CTA to make several safety changes.
An NTSB report said that train was able to make its way through five stop mechanisms because a "master lever on the operator console had been left in a setting that allowed the train car brakes to recover and reset." That allowed the train to start rolling over until it crashed into another train.
Transit officials say there are more than 80,000 daily riders along the Blue Line O'Hare branch.