That Sinking Feeling on the Underground

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On one of my trips into the City I noticed these yellow things on the Bakerloo Line platform, at Paddington. I was fairly certain they were something to do with lasers, but beyond that I was fairly clueless. Turns out there are some 1800 of these scattered around the Bakerloo part of the Underground and they were put in while the work on the new Elizabeth Line tunnels was ongoing. The yellow things house a retro reflector, the sort of thing that you see in modern reflective materials, which is designed to reflected light back to the source no matter the angle it hits at. The right hand photo shows, suspended from the ceiling, a ‘Total Station’ which is home to a device like a laser theodolite. Being robotic it moves up and down as well as around and measures the distance to the reflectors, with an accuracy of two millimetres. It takes measurements constantly, although it now seems to be switched off. The system is looking for any effects the work on the Elizabeth Line may be having on the existing underground network. Since that work is now finished, as the testing phase begins, that probably accounts for the device being switched off. All the information is fed into a central computer and if the measurements are too far out of kilter, engineering staff are sent to see how far the station has sunk. There is a similar set up at Bank Station, on the Central Line and that appears to still be working happily away.