SNCF officials said preliminary findings showed that a metal plate, bolted on the side of rails to keep them aligned, appeared to have become loose and moved inadvertently to the middle of a switch near the station, causing the derailment of several train cars.

"The plate moved right to the middle of the switch, preventing wheels from passing through normally," SNCF Chairman Guillaume Pépy said at a news conference.

Some experts questioned how a plate, normally tightened by four bolts with locking washers, could have become loose.

"That is the main question," Mr. Pépy said. "It's too early to make conjectures."

The switch had been inspected July 4 and maintenance workers hadn't noticed any problem, SNCF officials said, adding that a train had rolled on the same track about 20 minutes before the accident.