By Keith Laing - 03/25/14 09:29 AM EDT
Investigators believe the operator of a subway train that derailed at Chicago’s O’Hare International Airport on Monday fell asleep at the controls, the Chicago Tribune reports.
The Chicago “L” train blew through a bumper at the end of the tracks at the station for O’Hare, which is the second busiest airport in the U.S.
The train ended up halfway up an escalator leading to the airport terminal.
“I can confirm that she was extremely tired,” Amalgamated Transit Union Local 308 President Robert Kelly said, according to the paper.
“Indications are she might have dozed off,” Kelly continued.
The Chicago 'L' subway is the third busiest public transit system in the U.S. with an average of weekday ridership of 734,900, according to the American Public Transportation Association (APTA). The Chicago subway is only surpassed by the New York City subway and Washington, D.C.'s Metrorail in U.S. transit ridership.
The nickname 'L' is a reference to the fact that most of the Chicago transit system's downtown tracks being elevated, but the station at O'Hare is located underground.
The Chicago train derailment resulted in injuries to more than thirty passengers who were on board the train when it left the tracks.
The National Transportation Safety Board (NTSB) said on Monday that is investigating the accident.