Ok, I spent a lot of time on the train recently. These plates are on most platforms…. funny sliders…. are they plugged in…. what do they mean…. an answer….
“Datum plates are fixed to structures (platforms, bridges and overhead line masts, etc.) located near the track. The details on the plate specify the relative position of the track, so that it can be monitored for movement.
Figure 6 shows a datum plate identified as chainage point number (C.P.N°) 58. The letters “DN” (on a red background) denote that the information on the plate relates to the Down line. “Offset” is the horizontal distance from the plate to the running edge (“R.E.”) of the nearest rail of the relevant track, in this case 2915 mm. The track is canted at 35 mm. The top of the movable slider block is normally set at the level of the nearest rail. If it is necessary to fix the datum plate at a higher level than the track, the number above the slider block indicates the difference in height between the rail head and the slider block (300 mm in the example illustrated).
A red slider block means that the data on the plate refers to the actual position of the track at the time when the datum plate was installed. A green block denotes a track design position, to which the track should be returned in the event of it moving out of alignment.”
via Other Lineside Signs.