The head area of a rail is calculated as follows. For an unworn rail profile the head area is bounded by the perimeter of the head and straight lines drawn through the bottom of the rail head. For a measured rail profile the calculated head area is the intersection of the head area of the unworn profile with that of the measured head as shown below.
Head Area calculation
By intersecting the measured head with the unworn profile head the contribution of plastic flow to head area is minimized.