At least 49 people, including a child, were killed and 600 injured on Wednesday in a railway accident in Buenos Aires, Argentine officials said.
An eight-car train entered the Once station, one of the busiest in the capital, at 8.30 am at a speed of 26 kph and slammed into the buffers.
The train, which was heading from Moreno to Buenos Aires, was carrying between 800 and 1,000 commuters, the majority of them headed to work in the capital.
The accident may have been caused by brake failure, but investigators are waiting to take a statement from the train operator, who was injured in the crash, officials said.
Transportation Secretary Juan Pablo Schiavi said the cause of the accident was unclear.
"The train entered (the station) at the regular speed. That's the part where the accident happens, the mystery or the responsibility. We don't know what happened in the last 40 meters. The operator was at his post and the train did not stop," Schiavi said.
The accident's cause has not been determined and "investigative tasks are being conducted", Transporte de Buenos Aires, or TBA, which operates the train service, said in a statement.
Railway union leaders said the train involved in the accident was a 40-year-old or 50-year-old unit, adding that Argentina's railroad system was antiquated and lacked investment and maintenance.
Video cameras at the station took footage of the crash, showing the second car crushing the first and trapping dozens of people inside.
"We were getting ready to get off because the train was slowing down and we felt a sudden jolt and the second car looked like it was going to envelope the first one. The people around me were mashed together," a survivor who identified himself only as Marcelo said.
"Thank God that I can tell the story," the survivor, who was riding in the first car, said.
"I was standing and everybody fell down, everybody was desperate, we all wanted to get out, but we couldn't. It was like an earthquake," a female commuter said.
More than 100 ambulances and two helicopters took part in the rescue operation, transporting the injured to hospitals around the city.
Rescue teams worked for more than four hours at the station, cutting the tops of the train cars to reach victims.
At least 200 victims are in serious condition, officials said, adding that most of those injured in the accident sustained bruises, fractures, cuts and respiratory problems.
Hospitals across Buenos Aires are on alert to treat the injured, cancelling regular services.
Hundreds of relatives of people aboard the train are going from hospital to hospital in search of loved ones.
"I'm searching for my brother. He took the train today to go to work and never got to his job. We don't know where he is," a woman told a television station.
The Buenos Aires city government set up an information line and media outlets are broadcasting lists of the injured to help relatives track them down.
President Cristina Fernandez cancelled the only event she had scheduled for Wednesday, but the government did not say whether the move was in response to the disaster.
The accident is one of the worst involving a train in Argentina in the past 50 years.
February has been a dark month for Argentina's railways.
A crash involving two trains killed 236 people in Benavidez, a city in the Buenos Aires metro area, in February 1970.
In February 1978, 55 people died when a train entering Buenos Aires from Tucuman province hit a truck.