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Argentine commuters watched in horror as a train overshot platform after platform before slamming into a metal barrier at Buenos Aires' Once station, killing at least 49 people and injuring hundreds.

The impact crushed the front cars packed with commuters into a twisted pile of glass, metal and bodies. More than 1,200 people were on the train during the morning commute, and half of them were injured.

Authorities do not know yet what caused the accident but passengers said the conductor seemed to be having trouble with the brakes before the trained crashed.

The deceased include 48 adults and one child, according to Argentina's Police Commissioner Nestor Rodriguez. More than 600 were injured, including 461 who were hospitalized, Transportation Secretary J.P. Schiavi said.

Rescuers spent the morning trying to free dozens of trapped commuters from the wrecked train's first car. Emergency crews were forced to cut the roof of the train and retrieve victims using a pulley.

The impact of the crash drove the second car six metres into the first car.

Survivors described a scene of chaos as the fast-moving train careened into the platform. Windows exploded as the cars smashed and broke apart. Passengers reported being tossed onto the floor, into the air and into each other.

The collision marks Argentina's worst train collision since 1978, when a crash killed 53 people.

It's unclear what led to the collision but the head of the area train workers' union says the train had been checked by a mechanic the day before.

"This machine left the shop yesterday and the brakes worked well," Ruben Sobrero told Radio La Red. "From what we know, it braked without problems at previous stations."

He refused to speculate about what may have caused the crash.

Answers from the train's operator will have to wait, however, as the motorman has been hospitalized and hasn't been able to speak to his union yet.

Five serious train collisions have occurred in Argentina since Dec. 2010.

Schiavi defended the rail system at a news conference, saying other countries have accidents, too.

As he spoke, riot police squared off with angry protesters at the closed station.

The 28-year-old conductor had just started his shift, Schiavi said.

"Tiredness, his age, the problems that a conductor might face" are among the factors being investigated in the crash, he said. "This young person had just begun his shift moments before the accident."

Union leaders said there has been a failure to invest in new trains or upgrade old ones with modern safety equipment.

The train's last recorded speed was about 25 kilometres per hour.

"That's a very slow speed" for so many casualties, University of Southern California engineering professor Naj Meshkati told The Associated Press. "It's important to look at the age of the cars."

With files from The Associated Press