Doesn't matter what you say about length of distance or whatever, this is simply sheer incompetence. Whoever was working on that section left it with the rail there, knowing it was unsafe
I very strongly suspect that the person who left the rail there did so with the intention that they or someone else would move it later, but this was forgotten (if it was a long duration possession then it's possible that none of the people who knew it was there were on shift when the possession was given up) and if the line was checked in the dark it's possible that it was overlooked.
Possibly it was deliberately left in this position to make it obvious that the whole rail needed moving. Alternatively if the rail was being moved with a minimum number of people in the dark, nobody might have known it was in that position. As there is someone whose job it is to make sure the track is safe for trains to run, neither scenario is a problem in and of itself.
The person who handed this section back almost certainly didn't know the rail was there (if they
did know then there would very likely have been an intent to cause an accident and the police would be involved. As there is no mention of that in the BBC or RAIB reports then it is a near certainty that this has been ruled out).
The issue with the lengths of possessions and worksites is that the people responsible for ensuring that the line is safe cannot physically check the stretch of line they are responsible for themselves, and so have to rely on reports from other people, at least some of whom will themselves be relying on reports from others, and so on.
