At least 49 people have been killed and more than 600 injured in the worst train crash in Argentina in 40 years, officials say.
The train hit the end of the platform at Once station in the capital Buenos Aires during the morning rush hour.
"We assume that there was some fault in the brakes," Transportation Secretary JP Schiavi said.
Dozens of people were trapped for hours in the wreckage but all have now been successfully taken to safety.
The government declared two days of mourning and called off planned carnival festivities in the country.
Jumble of metal
"The train was full and the impact was tremendous," a passenger identified as Ezequiel told local television after the crash on Wednesday.
Medics at the scene were overwhelmed by the casualties, he added.
"People started to break windows and get out however they could," another eyewitness told Reuters.
"Then I saw the engine destroyed and the train driver trapped amongst the steel. There were a lot of people hurt, a lot of kids, elderly," the eyewitness added.
Police outside Once station had to "keep back the curious and concerned as paramedics treated the injured", eyewitness Tom English told the BBC.
The train had hit the barrier at about 12mph (20km/h), destroying the front of the engine and crunching the carriages behind it, Mr Schiavi said.
One of the carriages was driven nearly 6m (20ft) into the next, he added.
Survivors told local media that many people had been injured in a jumble of metal and glass.
Emergency medical system director Alberto Crescenti said that some passengers who survived had to have limbs amputated. Many suffered from arrested breathing and trauma to the thorax region.
Many are in a critical condition in the city's hospitals and there are concerns that the death toll could rise, the BBC's Vladimir Hernandez in Buenos Aires reports.
Five accidents have occurred in and around the city in recent months, our correspondent says.
Many parts of Argentina's rail network are antiquated and in need of repair and this incident will increase concern about lack of investment in the system, he adds.
"This is the responsibility of a company that is known for insufficient maintenance and... improvisation," Edgardo Reinoso of the train workers' union told Reuters.
"Lack of controls" on the part of state agencies was also to blame, Mr Reinoso added.
Trenes de Buenos Aires, the firm which owned the train, expressed its "deep regret" over the accident.
"The firm sends its condolences to all the families of those passengers who died and remains worried for the state of health of those who were injured," it said in a statement.
In September 2011, 11 people died when a commuter train in Buenos Aires hit a bus crossing the tracks and then hit a second train coming into a station.
This latest accident is Argentina's worst train crash since February 1970, when a train smashed into another at full speed in suburban Buenos Aires, killing 200 people.