It should not be possible for one person to make a mistake of this magnitude.
In case of a disconnected/off main safety system, the train should be limited on a hardware level to not be able to go over 40km/h.
Your suggestion about limiting speed after a shut down is a good one. On top of that, the rail operators should have spotted a trend of repeated safety system overrides and investigated why that was happening.
Exactly. Which means a whole bunch of other people allowed for this situation to occur. So it isn't one person responsible for this mistake.