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Dozens of passengers were injured on Thursday when a Chicago subway train smashed into a snow removal machine in the northern part of the city.

Officials said 23 people were taken to hospital, including three with serious but "non-life threatening" injuries.

The crash occurred around 10:35 local time (16:35 GMT), said Robert Jurhwicz of the Chicago Fire Department.

A total of 31 passengers were on the train along with seven Chicago Transport Authority (CTA) employees.

Keith Gray, assistant deputy chief paramedic, told reporters that four children were among the injured.

Officials said the snow removal device was traveling on the train tracks, but at a slower pace than the subway train, and that the two-car train rear-ended the plough.

It was unclear why the snow removal machine was on the subway track. Thursday was a dry, mild day in the area with a high of 19C (66F).

A CTA investigation was ongoing, according to local reports. The ´óÏó´«Ã½ contacted the authority for comment.

Chicago's public transport system is the second-busiest in the US, with an average of 1.6 million daily weekday rides on trains and buses.