The prosecutor’s office said the 30‑year‑old driver was arrested on suspicion of an “intentional breach of safety obligations” and that an investigation had been opened into the crash. The French army said the truck was carrying a military engineering vehicle – a mobile obstacle‑crossing system – that had been used in exercises in Belgium, adding that the driver was a civilian contractor. The collision happened at 6:48 a.m. between Béthune and Lens, near Bully‑les‑Mines, as the Dunkirk–Paris train, carrying about 243–250 passengers, was travelling at roughly 160 km/h.