A passenger train crashed through the end of the line early Monday at Chicago's O'Hare International Airport and barreled up an escalator and stairs, leaving 32 people with non-life threatening injuries, officials said.
The Chicago Transit Authority train, a mass transit train that ran on electricity, is expected to remain in place for at least a day while investigators from the National Transportation Safety Board try to determine how it jumped a bumper at the end of the line.
"The train is not going to go anywhere for the foreseeable future - it's not going anywhere today," NTSB investigator Tim DePaepe told a news conference.
Investigators will review station video of the train arriving and an outward-facing video recorder at the front of the electrified "L" train, along with signals and the train's condition, he said.
It was not immediately clear how fast the train was moving, but authorities were looking at speed as a possible factor, said transit authority spokesman Brian Steele.
"It's evident the train was going faster than it should," he said at the scene.
Neither the female train operator nor any of the passengers faced life-threatening injuries, said Chicago Fire Department spokesman Larry Langford.
DePaepe said the operator had been on duty for about six hours at the time of the crash. She was still being examined at a local hospital and had not yet been interviewed by investigators, DePaepe said.
Robert Kelly, president of Amalgamated Transit Union Local 308 that represents the operators, said the woman had worked more than 60 hours during the last seven days.
"The operator might have dozed off. She did indicate to me that she was extremely tired," said Kelly, who spoke to the operator after the crash.
"She doesn't have an explanation of how this happened."
Langford said the eight-car train jumped a bumper at the end of the line just before 3:00 a.m. Chicago time (0800 GMT).
It was not immediately clear how long train service on that part of the line will be suspended. Buses were shuttling passengers from O'Hare to the next train station, according to CTA officials.
Passersby gawked at the crash scene, with some people saying the incident made them a little more nervous about traveling by train.
"I feel like there's accidents all the time with the trains, but not this bad," said Meghan Cassin, 25, a Chicago resident who was heading to work after a trip to Florida. "They take corners really fast."
In September, an unmanned Chicago Transit Authority train collided with a standing train at a station in a western suburb of Chicago during the morning rush hour, injuring at least 33 people.