> - Since the system was frequently failing and proceeding at slow speeds causes severe operational slowdowns, the control center started allowing them to proceed at linespeed instead, possibly with some ad-hoc checks to make sure the track was clear. - That got normalized, and whatever ad-hoc checks (if any) they were doing failed in this case.

This is a 200km/h line, which is right at the limit of the speed where you don't need in-cabin signaling (plenty of lines in France run at 200km/h without it), which explains why a careless control center operator could end up in this spot.