It is difficult to tell what they are surveying, if it was for a buried cable they would probably have a CAT scanner (which in this case means Cable Avoidance Tool!) But even they only tend to work when there is current flowing, for example I had to ask a Station Manager to turn the lights on during the day to ensure we could detect the position of the lighting cables running through the platform.
It also does tend to look like the sort of surveying equipment that P-way would normally use, OLE surveys normally require just measuring from the running edge of the rail, for which a hand held laser or even a good old tape measure suffices. It is also good practice to use GPS to measure the exact location of a pile, as this can give accurate xyz coordinates which is useful for getting the correct ground level as well as horizontal position. All that is is a GPS receiver mounted on a stick, programmed to give the exact position of the end of the stick. It is very accurate, provided that the operator does not push it too far in to soft ground reducing the ground level by a few centimetres!
However by now all survey work really ought to have been 100% complete. It gets a lot more difficult to change the design to suit once half the piles are already in...