Rescue workers cut through a wrecked commuter train Wednesday to reach passengers who were crushed inside when it plowed into a retaining wall at a terminus, killing 49 people and injuring 600.

"In the first car, there was a human wall," a firefighter told AFP, likening the heaped up bodies to the scene of a 2004 Buenos Aires disco fire that left 194 dead.

Asking not to be identified, he said they had to break through the train's skylights to reach the dead.

Fernando Sostre, a police spokesman, said rescue workers had to cut through the roof of the first car because "it was very difficult to prize free the bodies."

"With a system of pulleys, a tripod and belts, we pulled the bodies out, and at the same time, we worked with spreaders and cutters to cut through the metal to reach the victims. We had to work very carefully so as not to cause further damage," he said.

Cries of "Red! Red!" went up each time a seriously injured victim was pulled out of the wreckage and raced in stretchers up the station stairs to the street outside.

Two medevac helicopters used a closed section of the street to pick up the critically injured and ferry them to hospitals.

Ambulances sped in and out of the station carrying away the less seriously injured, designated with the code "yellow."

Pedestrians approached to take pictures with their telephones, until police pushed them back.

The front of the train was the most affected. Carrying some 2,000 people during rush hour, the train apparently lost its brakes as it pulled into the Once station, the terminus of a rail line linking the center of Buenos Aires to the city's populous western suburbs.

Failing to stop, it crashed head on into a retaining wall, driving the second car seven meters deep into the lead car, Sostre said.

"I saw the train coming in a way that wasn't normal," said Alfredo, a 28-year-old who was on a platform waiting for a train going in the opposite direction. "I began to scream: 'It's not braking! It's not braking.'"

"I threw myself back when I felt the impact, it was like an explosion," he told AFP.

He said the train doors opened and he broke windows to rescue two women.

"Then I helped a man whose body was outside the train but whose legs were trapped inside. Behind him were two dead women," he said.

Rodrigo, a 23-year-old passenger, said he sustained blows to the back and the head, but after being treated, he returned to the station to get his car.

"The only thing unusual was that the train didn't brake, because we always travel this way, all heaped together. That's normal," he said.

Members of the civil protection force were trying to calm down family members who rushed to the station seeking news of their loved ones.

Four hours after the accident, and after evacuating a woman with an open fracture of her right leg, Alberto Crescenti, the head of emergency services in Buenos Aires, called off the rescue operations.

In the station, 49 bodies were waiting to be transported to the morgue for identification.