An Amtrak passenger train arriving in Chicago from Grand Rapids, Mich., slammed into the back of a stopped freight train this morning, injuring 71 people, five of them critically.

The collision occurred at 11:30 a.m. as the Amtrak Pere Marquette passed through a South Side freight yard alongside the Dan Ryan Expressway. The impact left the passenger train partially derailed, its mangled locomotive resting atop the back of a Norfolk Southern freight train.

Four people, believed to be Amtrak employees, were pinned in the wreckage until firefighters cut them free with hydraulic rescue tools.

The cause of the accident is under investigation, said Amtrak spokesman Derrick James. A team from the National Transportation Safety Board was expected to arrive in Chicago this evening to begin examining the wreckage.

The train carried 187 passengers and five crew. It departed Grand Rapids at 8:02 a.m., and was due in Chicago at 10:30.

Robert Addison, who was in a home a block away from the accident site at 51st and Shields, said he heard the trains collide.

"It sounded like a bomb, or something," said Addison. "Just a loud boom."

Cheryl Osentoski of Grand Rapids was aboard the train with her family, coming to Chicago to do some Christmas shopping.

"I was actually sleeping," she said. "We ran into something, and we hit it pretty good. A lot of people were thrown out of their seats. We were scared, and we prayed for everyone on board"

Osentoski said a number of the passengers in her car had broken bones and were bleeding from their injuries.

Ariel Hommes, 26, of Grand Rapids, said the accident happened after the train slowed as it drew closer to Union Station.

She rode in one of the rear cars, furthest from the impact, and said the passengers around her were not seriously injured.

"Everyone seemed totally shocked," she said. "My knees are a little sore."

Firefighters pitched ladders up the steep embankment separating the tracks from the surrounding streets, and helped many of those who were not seriously injured to nearby Parkman School, where they awaited buses to Union Station.

Other passengers were evacuated by stretcher to triage areas. A fire department spokesperson said 20 ambulances responded to the incident and treated victims.

Acting Chicago Police Superintendent Dana Starks said detectives had no indication that the incident was anything other than an accident.

An overhead camera near the scene feeds into the city's surveillance network and likely captured the accident, Starks said. Chicago police will turn that footage, as well as other evidence over to federal investigators.

The track is owned by Norfolk Southern, said company spokesman Rudy Husband. Norfolk Southern dispatchers control the signals and traffic on that section of track as well, Husband said.

He declined to say how far the closest signal was from the accident site, and what the Norfolk Southern train was doing on the tracks at the time of the crash. Two Norfolk Southern crew members were on the train. They were not injured, said Husband.

Federal Railroad Administration data shows Norfolk Southern had three train accidents this year in its Cook County operation, including two derailments.

Husband could not immediately say whether those incidents occurred in the vicinity of today's collision.

All told, the railroad logged 24 safety incidents and 20 injuries this year in the county.

The accident halted passenger rail traffic between Chicago and eastern destinations, said Amtrak spokesman Marc Magliari.

Magliari said he was not certain what the speed limit on the track was, how fast the Pere Marquette was traveling, or how long the train operator had been on duty when the accident took place.

James said the inquiry will focus on the condition of the rail traffic signals nearest to the accident, as well as whether the Amtrak train had the right-of-way.

The American Red Cross has set up a hotline for those seeking passenger information at 312-729-6200.