The operator of a train that derailed near Paris on Friday, killing at least six people, says a dislodged steel piece joining two tracks at a junction could explain why it came off the tracks.
SNCF said the piece, which was bolted on to the track, had moved, preventing the rolling stock from passing through.
"The reasons why this fishplate dislocated itself is the very focus of the investigations," said SCNF president Guillaume Pepy.
The company will now check the state of 5000 similar junctions throughout the French rail network.
Traffic remained disrupted on Saturday on the central train line linking Paris to Orleans, Limoges and Toulouse, SNCF said.
The train, a regional service that travels more slowly than express trains, derailed at the station of Bretigny-sur-Orge, 26km south of the capital at about 5.15pm on Friday. It was carrying about 385 people.
Workers spent the night cutting through tangled metal, but found no more casualties. A crane was brought in to lift a carriage that fell onto its side and others torn open in the accident.
Dozens of people were injured, nine seriously.
Essonne region prefect Michel Fuzeau said more bodies might still be found in the wreckage and under the tilted carriage but dogs had not detected any more casualties so far.
There have been several train crashes in France in the past few decades. One of the deadliest was in 1988, when a commuter train headed into the Gare de Lyon in Paris crashed into a stationary train, killing 56 people, after its brakes failed.
In 2002, a fire swept through the carriages of a Paris-Vienna overnight sleeper in eastern France, killing 12.