The train, travelling on the line between Aix-en-Provence and Paris, "pulverised" the blocks, even though they were "pretty heavy", according to Carpentras public prosecutor Pierre Gagnoud.
"There are no objective elements that would support a terrorism investigation at the moment," he told the AFP news agency. "But an investigation for wilful misconduct seems obvious."
By the time a railway employee arrived at the scene, two other blocks had been placed on the line, "which proves malicious intent", he said.
Finger prints and DNA traces have been taken, in particular from a bottle of alcohol found at the scene.
An investigation into "dangerous damage and the deliberate endangering of life" has been opened.