The crowded intercity train, leaving Paris at rush hour before a holiday weekend for the city of Limoges, jumped the tracks 20 miles south at Brétigny-sur-Orge station. The seven-car train split into two, with some cars riding up the station platform and flipping over.

Six people died, two were in critical condition and seven more were in serious condition, officials said; 21 others were still in the hospital. More than 190 people were treated at the site for lesser injuries.

Officials said the death toll might increase as the wreckage was removed because some people might be beneath the overturned cars.

Railway investigators discovered that a metal clip joining two rails as part of the switch, which guides trains from one track to another, had worked loose and disconnected from its normal position, said Pierre Izard, the director for infrastructure at the national rail company, S.N.C.F.

"It broke away, became detached and came out of its housing," Mr. Izard said at a news conference at the scene. "It moved into the center of the switch, and in this position it prevented the normal passage of the train's wheels and seems to have caused the derailment."