Two commuter trains collided near Chicago on Monday, injuring as many as 48 people.
The Chicago Transit Authority trains — one travelling eastbound and the other westbound — were on the same Blue Line track when an outbound train stopped at Harlem station was hit by an out-of-service train at about 8 a.m.
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CTA spokeswoman Tammy Chase said 33 people were injured and taken to nine hospitals, while Forest Park Mayor Anthony Calderone said 48 people were taken to 10 hospitals.
All injuries are said to be minor.
"Most of the people were complaining of either neck pain or back pain," said Calderone.
Locomotive engineer Martinez Butler, who works for another train company, was standing at the corner when the CTA trains collided.
"There was a train that was stopped at the station. Another train came through and looked like it blew the signal, because I heard beeping," she said. "I'm a locomotive engineer. I know the systems, I know the sounds. When you hear those beepings it's warning you that there is an obstruction in front of you and you need to stop."
Police are reportedly treating the area as a crime scene because the out-of-service train may have been stolen.
Authorities are investigating what led to the crash and why the out-of-service train was on the track.
Robert Kelly, president of the union that represents train operators, said the out-of-service train's safety mechanisms should have stopped it automatically before it entered Harlem station.
"Right now it is starting to look like a mechanical malfunction," Kelly said.