The train derailment that killed at least six people in central France on Friday may have been caused by a loose steel component at a junction, French train operator SNCF said.
The component, which should have remained bolted onto the track, moved to "the middle of the track junction", preventing the rolling stock from passing through, Pierre Izard, head of infrastructure services at SNCF, said on Saturday.
SNCF said it would immediately start checking some 5,000 similar junctions throughout the French rail network.
The accident, which injured dozens of people, marred festivities for France's July 14 Bastille Day, traditionally the cue for French families to embark on long summer holidays.
Workers spent the night cutting through tangled metal, but found no more victims.
A crane was brought to the crash site to lift a carriage that fell onto its side and others torn open in the accident.
The train, a regional service that travels more slowly than France's TGV express trains, veered off the track en route from Paris to the city of Limoges at the station of Bretigny-sur-Orge, 26 km (16 miles) south of the capital.
National rail operator SNCF said the train was carrying around 385 people and that an investigation was under way.
Transport Minister Frederic Cuvillier dismissed speculation the train was travelling too fast as it entered the station and said the accident could have been much worse if the train driver had not reacted quickly to avoid hitting another train that was only 200 metres (yards) away.
"There could have been a much more serious collision with a much heavier toll," Cuvillier told France Info.
Guillaume Pepy, head of rail company SNCF, pledged thorough checks of the network's switching systems starting on Saturday.
"We know that the SNCF's equipment needs to be renovated... but it is premature to make a link between the state of the infrastructure, the material used and the accident," Bernard Decoux, mayor of Bretigny, told French TV channel iTele.
"Everything was fine and then all of a sudden it was if we were riding through gravel," Clement, 17, one of the train's passengers, who was in the second wagon, told Reuters.
"Then the wagon in front of me started to tilt over."
The train crashed just a few days after the government unveiled details of planned investments in the railway network. Traffic was disrupted on train lines between Toulouse, Orleans, Limoges and the Paris-Austerlitz train stations, SNCF said.
France has suffered several train crashes in the past few decades. One of the deadliest was in 1988, when a commuter train headed into Paris' Gare de Lyon 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.
President Francois Hollande, due to give the traditional Bastille Day address from his Elysee Palace on Sunday, raced to the scene on Friday to commiserate with families of the victims.