49 CFR §213.339

1. (a)A continuous search for internal defects shall be made of all rail in track at least twice annually with not less than 120 days between inspections.
2. (b)Inspection equipment shall be capable of detecting defects between joint bars, in the area enclosed by joint bars.
3. (c)Each defective rail shall be marked with a highly visible marking on both sides of the web and base.
4. (d)If the person assigned to operate the rail defect detection equipment being used determines that, due to rail surface conditions, a valid search for internal defects could not be made over a particular length of track, the test on that particular length of track cannot be considered as a search for internal defects under § 213.337(a).
5. (e)If a valid search for internal defects cannot be conducted for reasons described in paragraph (d) of this section, the track owner shall, before the expiration of time limits—
   1. (1)Conduct a valid search for internal defects;
   2. (2)Reduce operating speed to a maximum of 25 miles per hour until such time as a valid search for internal defects can be made; or
   3. (3)Remove the rail from service.