The National Transportation Safety Board is investigating a subway train derailment at Chicago's O'Hare International Airport, the agency said on Monday.
A train on the Chicago "L" transit system's Blue Line derailed early Monday morning at its station at O'Hare airport, which is the second busiest airport in the U.S.
The NTSB said Monday that it was launching an investigation of the incident.
"NTSB launches to Chicago; a CTA train at O’Hare International Airport rail terminal ran through the bumper at the end of the track," the accident investigation agency tweeted.
"Blue Line service has been temporarily suspended between O'Hare and Rosemont due to a derailment at O'Hare," the agency said in a service delay alert.
"As a result, trains are operating between Rosemont and Forest Park (via downtown), only, at this time," the service alert continued. "Bus shuttle service is available to provide connecting service through the affected area between O'Hare and Rosemont."
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 train that derailed ended up on an escalator leading up to the airport, according to a report in the Chicago Tribune.
More than thirty passengers who were on board the train were injured when it left the tracks.
-This story was updated with new information 5:52 p.m.