They said the train was unable to stop, presumably due to faulty brakes, and it slammed into the buffers inside the centrally located "Once" station. More than 800 passengers were aboard the train, state news agency Telam reported.

A police spokesman said 49 people had been killed.

"The train entered the Once station at 26 kilometres per hour (16 mph)... we suppose there was some flaw in the brakes," Transport Secretary Juan Pablo Schiavi was quoted as telling Telam.

Hundreds of thousands of people travel into Argentina's capital from the suburbs every day. The dilapidated and overcrowded rail services, run by private companies and heavily subsidized by the state, are plagued by accidents and delays.

In September, two commuter trains crashed into a city bus, killing 11 people. And one year ago, four people died during another train accident.

(Reporting by Hilary Burke; Editing by Sandra Maler)

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