The first two cars of train No. 504 could be seen sitting about a 100 feet away from the rest of the train.
They were tilted off the track.
At least 20 ambulances, 12 fire trucks, and about 10 police squad cars were on the scene at about 9:15 a.m.
Some passengers were being treated on the scene. No information was immediately available on which hospital the injured were being taken to.
Passengers could be seen sitting on the side of the train next to the tracks.
Stephanie Smith, who was sitting in the second car in the upper tier, said she heard brakes screeching, then the train came to a halt.
"I just went flying into the safety seat bar and fell to the ground," said Smith, a master's student at DePaul University, told the newspaper. "We were skidding out of control. Weaving back and forth. People were weeping and screaming."
Cement appeared to have crumbled off the bridge at 47th and Federal Streets, where the train was stopped.
Firefighters were raising ladders to the elevated track. There was a 30-foot gap between two rail cars, and several broken windows.
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