The operator told Amalgamated Transit Union Local 308 President Robert Kelly that she had worked a lot of overtime recently and was "extremely tired" at the time of the derailment, he said at 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. No one suffered life-threatening injuries.
Earlier, National Transportation Safety Board official Tim DePaepe said investigators had not drawn any conclusions about the cause of the accident, but were looking into whether faulty brakes, signals or human error were factors.
The operator, who was still hospitalized, will be interviewed, DePaepe said, and investigators would examine her routine over the last few days.
The timing of the helped avoid an enormous disaster, as the underground Blue Line station is usually packed with travelers coming to and from Chicago.
Denise Adams, a passenger on the train, described the impact to reporters.
"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."
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.
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 traveling 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 Monday morning that most were able to walk away from the wreck unaided.
Chicago's 240-mile subway system, which had fallen into disrepair in recent decades, has recently undergone renovations. A four-year, $429 million overhaul has started on the Blue Line from O'Hare to downtown. The line, which still has stations built in the late 1800s, was extended to O'Hare in the early 1980s.
Transit officials say there are more than 80,000 daily riders along the Blue Line O'Hare branch.