I think you may have forgotten about the construction of New Haden, the initial deck that the track is laid on is large panels of 3/8" plywood covering all the baseboard area, these large panels initially forming the contours of the layout, where the track was to be laid the ply was first covered in 1/2" notice board material ( brown sh1t soft board ) then covered with cork.
It was the 1/2" notice board material that caused any undulations in the track bed,caused by being held down with large weights when fitted and the layout later being stored in a damp wet atmosphere causing the notice board material to expand when it got wet, the large weights causing depressions, the depressions being leveled before I laid any track having removed the previous track work, Levelling the track bed out is something that I have had to do on another layout recently