They are called 'Datum Plates'. They are used to moniter the relative movement of the track in relation to adjacent structures.
Top left number is the number of the datum plate, one below is is the Lateral Offset to the plate. i.e. The distance, in milimeters, from the structure to the running edge of the rail. The next one denotes the line. UM = Up Main, etc.
At the bottom is the designed cant of the track.
They are only used when the track runs closer than 3m from a structure, not on all electrified lines as mentioned above. If the plate was fitted with a Red slider, it shows the track condition at the moment the plate was installed, where as if it was Green it would show the designed track possition.
Where a route hase ABT (Absolute Track Geometry) the whole route must be fitter with datum plates, and the slider will always be green.
Hope this helps.
