HSTs are mostly computer-driven because you're past any incident before you see it, let alone react: at those speeds reaction times translate as quarter miles, and an emergency stop upwards of a mile. Few of us can see clearly that far, so sensors are essential and the drivers rely on nothing but.
The signalling was at two levels, HS-compatible and normal operation-compatible: the lower speeds were supposed to reflect that. However, the train's systems had only just been updated that morning and a close examination of the trackside film shows the front of the train started slowing before the rear, with the result that the rear power unit pushed the coaches in front of it and off the track, while the front unit remained on it at first. One can only surmise the system at the front and back were on different settings.
Sadly enough, the curve was only there to avoid trains running through the regional hospital!