Track Cross-Level Deviation and Warp Against a Class Limit

A rigid truck bridging a section that rises on one rail and falls on the other has one wheel carrying much less than its share, and a lightly loaded wheel on a curve with lateral force is the wheel that climbs. That is the mechanism, and it is why warp limits tighten faster with class of track than most other parameters do. The measurement detail that decides whether the reading means anything is the REFERENCE. On a curve the track is supposed to have cross level -- that is the superelevation -- so warp is deviation from the DESIGNED elevation profile, not from level. Two readings 62 ft apart on a curve designed for 4 in of elevation, measuring 4.6 in and 3.2 in, are plus 0.6 in and minus 0.8 in of deviation, and the warp is 1.4 in. Measured against zero instead, those same two readings happen to give 1.4 in as well -- but on a spiral, where the designed elevation is changing between the two points, measuring against zero reports the intended runoff as a defect: 4 in of elevation running off over 200 ft is 1.24 in of designed change in 62 ft, read as warp that is not there. The same twist is also reported scaled to a 31 ft base, because the two measurement lengths carry different limits. This does not ship the FRA limit tables. They are set by class and by parameter in 49 CFR 213 and the adopted regulation governs, so the applicable limit has to be entered. It does not evaluate the other geometry parameters -- gauge, alignment, surface, and the combinations of them -- each of which has its own limits and any of which can independently restrict speed, and it does not handle the special limits that apply within a specified distance of a joint, on a bridge, or through a turnout. It does not address the qualification and frequency requirements for track inspection, the recording of exceptions, or the remedial action a defect requires, all of which are regulatory obligations rather than calculations. Track geometry defects are a derailment hazard: 49 CFR 213, the qualified track inspector, and the track owner govern.

deviation at a point = measured cross level - designed cross level; warp = deviation at A - deviation at B; margin = entered limit - absolute warp; the same twist scaled to a 31 ft base = absolute warp x 31 / distance.

The cross-level and warp definitions, referenced to the DESIGNED cross level, by name, with 49 CFR 213 named as the source of the limits by class of track. The limit tables are not reproduced; the applicable limit is entered. The qualified track inspector and the track owner govern.

49 CFR 213 is US federal regulation and public domain; this is subtraction on the user's own level-board readings against a limit they supply.

Estimate. AHJ and licensed professional govern.

Field names used by the API: measured_a_in, designed_a_in, measured_b_in, designed_b_in, distance_ft, warp_limit_in, deviation_a_in, deviation_b_in, warp_magnitude_in, margin_in, warp_per_31ft_in

• The reference is the designed cross level measuring against zero on an elevated curve reads the superelevation itself as a defect49 CFR 213
• Measurement length matters 31 ft and 62 ft warp carry different limits; the length must be stated with the reading49 CFR 213
• Limits are entered, not shipped the class limits are set by the adopted regulation and must come from it49 CFR 213