I know zilch about high-speed train systems, but when a moving line (of anything) goes at speed round a bend, the centrifugal force makes the final sections pull outwards and the thing 'lashes its tail', ie the end leaves the bend. From watching the video, this seems to have happened. The driver needs support and counselling ( and IMO further medical treatment for delayed shock and post-traumatic stress) I see there's been another train accident, in Switzerland this time.