Where-
Am = measured alinement deviation from uniformity (outward is positive, inward is negative).
AL = allowable alinement limit as per §213.327 (always positive) for the class of track.
Sm = measured profile deviation from uniformity (down is positive, up is negative).
SL = allowable profile limit as per §213.331 and §213.331 (always positive) for the class of track.
49 C.F.R. §213.332