The operation is an "extraordinarily difficult technique given that we are in a train station," Pepy said. "For the moment, we don't know how long it could take." He said the operation could last through Sunday, which is the July 14 Bastille Day holiday, and into Monday, stressing the crane's operators will be careful and slow in lifting the cars.It was not immediately clear whether the damaged cars would be lifted over buildings onto trucks as authorities had indicated â¿¿ or whether the debris would be taken away by rail. There was no immediate sign that the damaged car that was lifted to check for victims had left the tracks. Pepy, the train authority chief, said investigators found that a 10-kilogram (22-pound) piece of metal he compared to a staple between two rails in a switching system, which guides trains from one track to another, seems to have "detached itself from the rails, lifted and constituted the initial cause of the derailment." Investigators were looking into how this happened since another train had traveled safely through the station about 30 minutes before. In addition, they were trying to determine why the train's third car was the first to derail. Pierre Izard, another SNCF official, said the metal piece "moved into the center of the switch and in this position it prevented the normal passage of the train's wheels and it may have caused the derailment." Although for now it appears track failure was the cause of the crash, Pepy added: "There can be no (definitive) answer in a few minutes, in a few days." He also said that all of the approximately 5,000 metal pieces on switching systems around France will be checked. The train was about 12 miles (20 kilometers) into its 250-mile (400-kilometer) journey to Limoges. Passengers and officials in train stations throughout France held a minute of silence at noon to commemorate the accident. Hundreds of thousands of people were expected to take trains this weekend to the coast and mountains and to see family. Summer weekends are always busy on France's extensive rail network, but this one is typically one of the busiest because of Bastille Day.