Because the weakest link is people.
The system will fail safe and the interlocking will prevent a conflicting move. The human aspect is where the system fails. When people question and fall into casual conversation it leads to mistakes and lack of concentration. Which is why we keep conversation to be "Safety Critical Communication" only.
As an example I know of an incident where someone was authorised past a signal into a station. The conversation went along the lines of "you are authorised to pass signal x at danger then you are clear into station" Unfortunately there was another signal before the station, also at red.
We have had trains authorised past signals but the wrong train was given authority and as no-one had confirmed the headcodes.
Lack of understanding to how far the movement is to take place. We had a SPAD onto a mainline as the Driver believed he was being authorised to carry out the shunt move but he wasn't. There was an expectation for him to stop at the position light.
There was almost a head on collision because there wasn't a clear understanding as to which signal was allowed to be passed because the Driver was in one place and the Signaller thought they were somewhere else and there was confusion over which signal.
I have heard stand up blazing rows on the radio tapes where Driver/Signaller have had a disagreement or the Driver has questioned the Signallers actions.
We have even had trains authorised past a signal by the incorrect signaller because the CSR connected to the wrong box.
People do not fail safe.
It is not my place to question if the 0834 is in front or behind me, or usually passed the junction first. My job is the safety and running of my train and I have full confidence that when given authority by the one person who is responsible for the signal then I happy to accept that.
With a PoSA it is an absolute.
With this incident if we accept the premise that the system was working correctly then human error has caused the crash. If we accept that all the humans involved carried out their duties to the letter then the system caused the crash. It is reasonable to accept the first premise.
Many incidents are due to communication errors.
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I think the problem here might have been (and we will no doubt find out in due course) that the Zs1 was given only to one of the drivers; the other one got a green and was proceeding at linespeed.
If both were driving on sight I would not have expected any collision, or at worst a minor prang.
Potentially I agree. I don't know what is interlocked and what isn't or from viewing the video if there even is a Zs1 signal. Even if it is what we both believe then the line wasn't checked and authority to pass was given whilst a train was in the section. That is not a failure of the system. It will be human error.
Or one had a SPAD but then INDUSI should have intervened.
My other suspicion is that the signals for the level crossing are only interlocked for the crossing (again doubtful) but one of the units must have passed the level crossing signal for some reason. Again it sounds like human error there.
My understanding is that being given the Zs1 means 40kmp as far as instructed/expected (next signal ?) and that would have been the crossing. The other train would/should of had a red. More human error by the sounds.
Again we would need confirmation from those who know that before being given the Zs1 the line must be checked.