As many as four dozen people were injured when two Chicago commuter trains crashed in a western suburb during Monday morning rush hour, though none of the injuries appeared to be life threatening, officials said.

The head-on collision happened at 7:45 a.m. CT on the Blue Line — part of the Chicago Transit Authority's 24-hour rapid transit service between O'Hare International Airport and the Forest Park terminal, the western terminus of the line. The Blue Line runs through downtown Chicago. 

The collision occurred near Harlem Avenue and Interstate 290 when a westbound train full of commuters was struck by an out-of-service and apparently unmanned eastbound train that was supposed to be parked and secured behind multiple fail-safe switches at Forest Park station.

"We were stopped at Harlem and then we hear like a big boom sound and everyone started flying out of their seats," said passenger Lyneisha Fields. The 18-year-old said she was taken to a local hospital's emergency room because she hit the back of her head on a metal bar in the train.

Chicago Transit Authority spokeswoman Lambrini Lukidis said 33 people suffered non-life threatening injuries.

Forest Park Mayor Anthony Calderone said he was told as many as four dozen people were injured, most reporting back and neck pain, and were taken to local hospitals. One of those requesting medical attention was a train operator.

"None were serious injuries thank god," he said.

CTA says said the train was undergoing repairs when it rolled away. The transit authority said an investigation into the collision has been started.

Lukidis said video from the scene shows no driver at the controls of the eastbound train. 

The run-away train was travelling just over 30 kilometres per hour when the collision happened, according to CTA. 

The CTA and Forest Park police said no criminal activity or foul play is suspected, according to a report by the Chicago Tribune. Forest Park is about 16 kilometres west of downtown Chicago

Chicago train crash

The two Blue Line trains collided in Chicago's west suburban Forest Park community. (@pulchritudino/Twitter)

With files from Associated Press