The derailment of a Chicago subway train at O'Hare International Airport last month caused $9.1 million worth of damage, according to the National Transportation Safety Board.
The NTSB released the finding in its preliminary report on the March 24 crash of a train on the Chicago Transit Authority's Blue Line at the subway station for O'Hare Airport.
The accident involved a train entering the station too fast and running through a stopper at the end of the tracks. The train eventually came to rest halfway up an escalator that leads to the airport terminal.
"The train operator told investigators that she had fallen asleep as her train entered the station," the NTSB said. "The last signal that she recalled seeing was a yellow-over-red signal aspect, which indicated the next signal would require her to stop the train. She said that she awoke as the train crossed the fixed trip stop, which was just beyond the stop signal."
The NTSB said the driver of the Chicago train that derailed, who has since been fired, also told investigator that she was tired before she began working her shift.
"She told investigators that she had inadequate sleep on Saturday," the accident investigation agency said. "She also said that she had felt tired when she started work on Sunday evening, which was just hours before the accident."
More than 30 people who were on board the train at the time of its derailment were injured when it left the tracks at O'Hare, but no one was killed in the accident. Federal investigators have said that they believe the damage would have been a lot worse if the accident occurred later in the day because O'Hare is the second busiest airport in the U.S. and the Chicago "L" is the third-busiest pubic transportation system in the country.
The "L" nickname for Chicago's trains is short for elevated. It is a reference to the fact that most of the system's tracks in downtown Chicago are above ground, though the O'Hare Airport station is underground.
The Chicago transit agency has made changes to its scheduling rules since the O'Hare derailment, including capping the number of hours its operators could be driving trains at 12 hours in a 14-hour period and requiring that they get at least 10 hours between shifts.
The full NTSB report on the Chicago train derailment investigation can be read here.