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Turns out that they are reflectors But (and how cool is this?) they are to reflect laser beams and are being used to detect if works above ground are shifting the shape of the tunnels (that scares me just a bit).
Originally posted by davespanners
Is this what you mean?
That's one of the reflectors near ground level. The others are on a simple (short) rod and bolted to the wall.
There must be a total of dozens of them.
Your link claims these are "relector generators", which is nonsense. They are either lasers (generators) or reflectors.
This one appears to have no visible power supply, and is perhaps built like this as it is within reach of passengers.
This image shows the low level reflectors and the high level ones.
edit on 31-3-2012 by ommadawn because: Additional comments.