A fishplate used to join two lengths of track has been blamed for the fatal train crash on the outskirts of Paris on 12 July.
The crash happened at 5.15pm local time last Friday.
A passenger train carrying around 380 people south of the French capital to Limoges derailed 200m from Bretigny-sur-Orge station.
It then crashed into the station, partially demolishing a platform. Six people died.
Images from the crash scene rekindled memories of the 2002 Potters Bar crash in north London when a northbound trainat high speed leaving carriages wedged between the station and the platform, killing seven and injuring 76.
Initially social media speculated that French crash had, like the Potters Bar disaster, been caused by a loose stretcher bar. Stretcher bars separate track at points.
However, a spokesman for France’s national rail company SNCF dismissed the suggestion.
“The cause of the crash was an éclisse,” he said, “a piece of metal which links two pieces of track.
“The éclisse went out of its position and settled in the points,” he said. “It is not a problem of the points.”
The failure of the 10kg eclisse, which the spokesman said was less than 500mm long, has kicked off an extensive inspection programme encompassing all 5,000 similar joins used throughout France’s national rail network.
The crash is being investigated by legal authorities and French safety authority BEA.