Praising the quick reflexes of the driver, who sent up the alert that halted all train traffic in the area, Transport Minister Frederic Cuvillier virtually ruled out human error in the disaster, saying the probe would focus instead on the "rolling stock, infrastructure and the precise signalling area".
"Fortunately the locomotive driver had absolutely extraordinary reflexes by sending the alert immediately, which avoided a collision with a train that was coming the other way and just a few seconds later would have smashed into the cars that were derailing. So it's not a human problem," Cuvillier told French radio today.
Rescue teams worked through the night checking the wreckage of overturned carriages to see if any passengers remained trapped inside.
Cuvillier said earlier Saturday that further "unfortunate discoveries" could not be ruled out.
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He also told French television that work to right the overturned train cars could take a "very long time (because) the carriages are very intertwined."
The bodies of five of the six dead have been extracted from the wreckage, said a source close the operation.
The regional train was heading from Paris to the west-central city of Limoges. It derailed as it passed through the station at Bretigny-sur-Orge, about 25 kilometres south of Paris.
Four carriages of the train jumped the tracks, of which three overturned. One carriage smashed across a platform and came to rest on a parallel track; another lay half-way across the platform.
Passenger Marc Cheutin, 57, told AFP he had to "step over a decapitated person" after the accident, to exit the carriage he had been travelling in.