49 C.F.R. § 299.313 – Track geometry; performance based.

(a) For all track of Class H4 and above, vibration in the lateral and vertical directions measured on the carbody of a vehicle representative of the service fleet traveling at a speed no less than 10 km/h (6.2 mph) below the maximum speed permitted for the class of track, shall not exceed the limits prescribed in the following table:

Table 1 to Paragraph (a)
Carbody acceleration limits 1 2
Lateral vibration 3	Vertical vibration 3
≤ 0.35 g peak-to-peak	≤ 0.45g peak-to-peak
1 sec window	1 sec window
excluding peaks < 50 msec	excluding peaks < 50 msec
1 Carbody accelerations in the vertical and lateral directions shall be measured by accelerometers oriented and located in accordance with § 299.337(c)(3).
2 Acceleration measurements shall be processed through an LPF with a minimum cut-off frequency of 10 Hz. The sample rate for acceleration data shall be at least 200 samples per second.
3 Peak-to-peak accelerations shall be measured as the algebraic difference between the two extreme values of measured acceleration in any 1-second time period, excluding any peak lasting less than 50 milliseconds.

(b) If the carbody acceleration requirements are not met on a segment of track, the segment of track is to be reclassified to the next lower Class of track for which it does meet the requirements of this part.