REPORTING FROM BUENOS AIRES -- An Argentinian federal judge promised to lead an investigation into the causes of a stunning train crash that killed at least 49 people and injured more than 600 Wednesday morning.

No official cause of the accident here had been determined by midday. "We imagine there was a problem with the brakes," Transport Secretary Juan Pablo Schiavi told reporters after federal Judge Claudio Bonadio said he would lead the inquiry.

Operator TBA issued a statement apologizing for the accident and promising to pursue all means of investigation to "promptly clarify" its cause. Several commuters interviewed at the station where the wreck occurred said they were familiar with the line and that it is notorious for its shabby condition.

PHOTOS: Train accident in Buenos Aires

The train that crashed was on the Sarmiento line, which brings commuters from the western regions of Buenos Aires to the center of the capital. It slammed into a barrier at the peak of rush hour. After impact, many cars pancaked or jumped the tracks, killing passengers and people waiting to board.

"This morning was like any other at the restaurant when I heard an explosion like a bomb that shook the entire station," said Carlos Alberto Ruiz, who works at a terminal fast-food stall about 35 feet from the crash scene. "I looked at Platform 2 and I saw a lot of dust and hurt and shaken people climbing out of train windows and doors."

Ruiz said rescuers cut a hole in the roof of a derailed train car to extract victims.

One passenger who identified himself only as Emanuel told the newspaper Clarin that he was nearing the end of his commute when he felt a strong impact, followed by passengers "falling on top of each other" and desperate screams.  

 "I travel every day on this train, but this morning I arrived after the accident for pure luck," homemaker Lourdes Heugemile said. "Nothing surprises me because this is the worst of all the train lines. It's always filthy, overcrowded and unsafe."

Several members of a rail workers union said in TV interviews that the commuter train system had fallen into disrepair. Yet union spokesman Ruben Sobrero told reporters that the braking system had been checked at a maintenance facility as recently as Tuesday night.

The Once station, where the wreck occurred, was closed shortly after the accident, and trains were diverted elsewhere. Journalists were not allowed to enter, and blue sheathing was hung to keep dozens of reporters clustered inside the terminal from viewing the crash scene.

Television coverage showed scenes of anguish and desperation among people arriving at the terminal seeking news of friends and relatives on the train. Observers could see victims being carried out in black body bags. Several hospitals were coping with the estimated 600 injured.

Police spokesman Nestor Rodriguez said the toll could have been much higher if the train, which reportedly was traveling about 15 mph when it crashed, had been going any faster.

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-- Andres D'Alessandro in Buenos Aires and Chris Kraul in Bogota, Colombia.

Photo: Injured people await transport after a train accident in Buenos Aires on Wednesday. Credit: Martin Quintana / EPA