The crash happened during a test run in the town of Eckwersheim, on a new Paris-Strasbourg line.
The train was carrying 49 railway technicians when it derailed, caught fire, and plunged into a canal. Eleven people suffered serious injuries.
The derailment was due to "excessive speed", officials say.
Pictures showed the locomotive partly submerged. Wreckage was also scattered in a field beside the track.
Reports say the Paris-Strasbourg high-speed line is scheduled to open next year.
Junior transport minister Alain Vidalies and Ecology Minister Segolene Royal will visit the scene, French media reported.