A six-year-old child died Monday, run over by the RER C after descending on the track in Egly, in Essonne, corroborating sources said.
In the early evening, the child, who lived near the Egly level crossing, a small town 40 kilometers south of Paris, very briefly escaped the supervision of his 15-year-old brother.
He got off the track as the train arrived, said the Evry prosecutor's office, confirming information from the gendarmes.
An investigation for "manslaughter against X" was opened in the wake.
"The preferred track is that of the accident," said the prosecution, stressing in particular that the RER C had followed its "classic circuit" and that "the sound signal had worked".
An autopsy is due to take place on July 19.
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Paris miscellaneous facts