CHICAGO (AP) — An Amtrak train from Michigan 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.

The damage to the passenger train was mostly to its engine, where some of the five Amtrak workers aboard were, authorities said. It was unclear how many of them were hurt. The train's three double-decker passenger cars remained upright.

No one was in the section of the Norfolk Southern freight train that was struck, and neither of the two workers aboard was hurt.

Amtrak passengers, many carrying winter coats and luggage, streamed off the train with the help of rescue workers. Some held the hands of children; others were taken away on stretchers and backboards.

Coert Vanderhill, 60, of Holland, Mich., said the train was approaching the station at 15 to 20 mph when the engine "just ran right up the tail end" of the freight train.

"Everybody just hit the seat in front of them," he said.

Vanderhill, who had come to Chicago to visit his children, had a small cut on his nose and said most of the other passengers also had minor injuries.

Two dozen Mary Kay representatives from Michigan were on the train as part of an annual shopping spree to Chicago, said Jane Studrawa, national sales director for the cosmetics company. Studrawa said one of the women was sent to a hospital with unknown injuries.

"They were shaken up," Studrawa told The Grand Rapids Press of the women, who were from the Grand Rapids, Midland, Mount Pleasant, Holland and St. Joseph areas.

Pam and Fred Zuke of Grand Rapids were traveling with their 5-year-old granddaughter, Gabrielle Ashton, to visit the American Girl doll store when they were thrown from their seats in the crash.

"We're kind of shaken up here," Pam Zuke, who said all three were bruised in the crash, told the newspaper.

University of Chicago Medical Center got 13 patients, three of them in serious to critical condition, spokesman John Easton said. He said none of the patients had injuries that appeared to be life-threatening, and he expected most to be treated and released.

Cook County's Stroger Hospital cared for another 25 patients, all with minor injuries, spokesman Sean Howard said. At Advocate Christ Medical Center, spokeswoman Deb Song said six of the 10 patients there were to be treated and released, and the rest were in fair or stable condition.

The Amtrak train was en route from Grand Rapids, Mich., to Chicago. The freight train was traveling from Elizabeth, N.J., to Chicago.

Norfolk Southern spokesman Rudy Husband said he had no details about what caused the accident or what the freight train was carrying.

Amtrak spokesman Derrick James said it was unclear which train was in the wrong place. He said Amtrak shares the track with Norfolk Southern, which owns it. He also said he did not know if the freight train was moving or stationary when it was struck.

Amtrak was awaiting information from an event recorder, a device similar to the black boxes on airliners, James said. Of particular interest was what the signals were before the Amtrak train got to the spot where the collision occurred, he said.

A team of nine NTSB investigators had arrived in Chicago to conduct interviews with crew members, passengers, dispatchers and supervisors in an investigation that could take months, Sumwalt said at a news conference.

"We're here to find out what happened so we can keep it from happening again," he said.

Investigators are particularly interested in finding out how both trains ended up on the same track at the same time, Sumwalt said.

"That's going to be a large part of our investigation," he said.

A police surveillance camera mounted on a nearby post recorded the accident and the footage will be turned over to investigators, Interim Police Superintendent Dana Starks said.