Okay so the dark patch is possibly a depression - that kinda explains that, although I'm not 100% convinced due to the wear marks either side, the regular but irregular nature of the 'depression' and the fact that the dark patch with a light border can look 3d due to the way our eyes process the information even though it's a 2d surface.
This still doesn't explain why the wear marks are even throughout that front section and then drop off to zero (effectively) at a sharp edge. This is NOT explained by rake, which would give a linear reduction in wear, and can only be explained if that front section of the plank is running parallel to the track with the rest being raked.
My maths may be a little rusty but I believe that if the front section of the plank is 1m in length and the plank varies in thickness by 2mm, and presuming that the wear on the plank is linear (I believe it is not), then the maximum rake the car could be running is 1.1 degrees. Any more would have to be taken up by flex in the plank.
Also, presuming that wear is linear, there wouldn't be any margin for there to be a depression in the plank that far from the front.