A train undergoing a trial on a new high-speed line from Paris to Strasbourg derailed on Saturday near the German border, killing 10 people, said the local prefecture.
Another 32 people were injured, 12 seriously. Five more people were unaccounted for, as it was not clear whether they had actually boarded.
Environment Minister Segolene Royal said at the scene the 49 people believed to be on board the trial run included not only staff from the SNCF railway but also their family members and other guests.
The accident was caused by excessive speed, although it was too early to say why the train was going that fast, said the prefecture.
Royal also said sabotage had been all but ruled out. But the accident jarred nerves, coming less than a day after gunmen and suicide bombers killed 129 people in waves of attacks in Paris.
Pictures from a photographer at the scene of the train accident showed the locomotive partly submerged in a canal alongside the tracks with other parts lying broken and detached in a field beside the track. Medical units including police divers were on the scene.
The second section of the Paris-Strasbourg high-speed TGV line on which the crash happened is set to open for service in April 2016.
