Some 385 passengers were aboard the train to Limoges when a joint bar that held two rails together is believed to have come loose, Radio France International reported Saturday.
Four cars jumped the tracks at the Bretigny-sur-Orge station just after 5 p.m. Friday, with three of the cars overturning, officials said.
Some 192 people were treated for injuries. The most seriously injured were airlifted to area hospitals.
Transport Minister Frederic Cuvillier said more victims could still be found.
Cuvillier said the accident was "not a human problem." He praised quick actions by the driver that avoided a collision with another train that was due in the station only moments later.
"Fortunately the locomotive driver had absolutely extraordinary reflexes by sending the alert immediately," the minister said.
French President Francois Hollande cautioned against "unnecessary speculation" as he visited the scene shortly after the accident. "What happened will eventually be known and the proper conclusion will be drawn."
Two cars derailed first, knocking four other cars off the track, said Guillaume Pepy, president of the national railway company SNCF, The New York Times reported.
"Some cars simply derailed, others are leaning, others fell over," he said.