Chicago ? An Amtrak train plowed into the back of a freight train and crushed one end of a boxcar under its wheels Friday, injuring dozens of people, some seriously. Most of the 187 passengers walked away unhurt.

Passengers were hurled into the seats in front of them in the accident on the city’s South Side, and four people had to be pulled from the front of the train, where the engine was located, authorities said.

The collision sent 71 people to a dozen hospitals, most of them in stable or good condition, according to the Chicago Fire Department. Three people were expected to be hospitalized overnight, said National Transportation Safety Board Vice Chairman Robert Sumwalt.

The cause of the wreck was not immediately known.