Help:Surveyor Topology Plateau
From TrainzOnline
Plateau (P)
Plateau (P) creates a plateau at the current height of the compass. Click LMB on the side of a hill to create a plateau at that level by raising or lowering the terrain.
This tool is affected by the sensitivty dial. Moving your cursor around over pre-existing hills will create a smoothing effect. Adjust the sensitivity to flatten/smooth areas quicker or slower.