CléonInvestigators found a fragment of rail on the track.
44 people were injured, one critically, among 186 passengers aboard.
The Rouen–Caen regional train derailed near a Renault plant, prompting a 130-firefighter mass-casualty response in France.
Rouen prosecutors have named no suspect and set no timeline to reopen the line.
Sources: Yahoo, Al Jazeera Online, South China Morning Post, TRT World