With regards to the NDL signage, I would not be surprised if these were related to water jetting, Sandite applications or perhaps some experimental measure to reduce wheelslip. The line is notorious for exceptionally poor railhead conditions and it is by no means unheard of for units to become stuck for hours on end in winter. Weedkiller trains (crewed by SWT drivers) run regularly, too.
If this is the case, I would expect the signs are perhaps to mark gradients or other areas which are particularly in need of special treatment, perhaps for water jetting at various intensity levels. The force of water blasting the prevalent leaf debris from the rails can be very effective in this area, and it's noticeable when an RHTT hasn't provided this type of assistance.
The MPVs used on RHTT workings in this area are usually equipped with pretty bright headlights, and I should imagine any drivers who were newer to the route could find meaning in the signs - especially the shape - in the hours of darkness, which would often be appropriate. I know this would not be ideal, but it might be more useful than a GPS-based reminder system, especially as A-GPS technologies can be a little sketchy in the area.
