I don't mean to absolve people for negligence, but when margins are very small and paychecks may have become a necessity to survive on this planet, sometimes within the complex process the collective behaviour gets pretty weird.

There are not that many people who (care to) see the big picture(s) and the consequences their semi-small actions may have in the long run.

(Un)Fortunately, finding someone who is "at fault" generally helps neither in investigating root causes nor in preventing similar problems in the future.

Usually it's detrimental, hence (criminal) investigations of personal liability in such cases are often only allowed after systemic investigation has concluded.

In court, you are allowed to withhold information that makes you look bad, which is simply lethally unhelpful when we want safer trains.