FILE – In this March 24, 2014 file photo, a Chicago Transit Authority train car rests on an escalator at the O’Hare Airport station after it derailed early in the morning, injuring more than 30 people, in Chicago. (AP Photo/NBC Chicago, Kenneth Webster, File)

Photos of the Chicago Transit Authority subway train that came to rest on an escalator shortly after it derailed early Monday morning went viral. More than 30 people were injured in the accident, though none seriously. Now, new details are coming out about the cause of the accident.

The Associated Press’s Jason Keyser reports that the train operator acknowledged that she dozed off before the accident. Even more distressing, she told investigators that she had fallen asleep at the controls one other time recently — overshooting a station platform.

Writes Keyser: “In Monday’s accident, which injured more than 30 people, she woke up only as the eight-car train jolted onto the platform and barreled up an escalator leading into the airport. The accident occurred around 3 a.m., as the driver was nearing the end of her shift. The woman had an erratic work schedule and investigators were looking to see if that played a role in her evident fatigue.

“She did admit that she dozed off prior to entering the station. She did not awake again until the train hit,” National Transportation Safety Board investigator Ted Turpin said at a final on-site briefing at the airport. Nearby, workers with electric saws and face shields were cutting up the lead train car, sending bright orange sparks flying, as they prepared to remove the wreckage.”

Fatigue among transit workers, particularly those who work early morning hours in “safety critical” positions as bus drivers and train operators, has long been an issue at transit agencies across the country.

Metro officials took steps to limit the number of hours some rail employees could work following a 2011 study that found those who worked in safety-critical jobs worked longer hours than allowed and that there were no limits on the number of consecutive days worked. In 2013, the transit agency launched a study to examine the effect of sleep habits on its workers.

Steven R. Hursh, president of the Baltimore-based Institutes for Behavior Resources, the firm that was working with Metro on the study, said the number of hours an employee works is not the only factor that contributes to fatigue. Problems can be aggrevated by failure to get enough sleep and by an employee’s work schedule. Workers with late-night shifts can be particularly vulnerable to fatigue because they are awake during hours when people typically get the best sleep, he said.

While federal regulations limit the number of hours a rail engineer, truck driver or pilot can operate a railroad train, a truck or a plane, but there are no such rules for transit workers. At Metro, the maximum number of hours an employee can work is set by the collective-bargaining agreement with the union. Metro officials asked for $5.5 million in fiscal 2014 to fund a second phase of fatigue-management efforts. Richard Sarles, the agency’s general manager, called the study a “major step.”