49 C.F.R. § 213.332
(b) For the conditions defined in paragraph (a) of this section, the combination of alinement and surface deviations for the same chord length on the outside rail in a curve and on any of the two rails of a tangent section, as measured by a TGMS, shall comply with the following formula:

Where— Am = measured alinement deviation from uniformity (outward is positive, inward is negative). AL = allowable alinement limit as per § 213.327(c) (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(a) and § 213.331(b) (always positive) for the class of track.

[78 FR 16107, Mar. 13, 2013]