49 CFR §230.98

1. (a)Condemning defects. Driving, trailing, and engine truck axles with any of the following defects shall be removed from service immediately and repaired (see appendix A of this part for inspection requirements):
   1. (1)Bent axle;
   2. (2)Cut journals that cannot be made to run cool without turning;
   3. (3)Transverse seams in iron or steel axles;
   4. (4)Seams in axles causing journals to run hot;
   5. (5)Axles that are unsafe on account of usage, accident or derailment;
   6. (6)Any axle worn 1/2 inch or more in diameter below the original/new journal diameter, except as provided in paragraph (a)(7) of this section;
   7. (7)Any driving axles other than main driving axles with an original or new diameter greater than 6 inches that are worn 3/4 inch or more in diameter below the original/new diameter.
2. (b)Journal diameter stamped. For steam locomotives with plain bearings, the original/new journal diameter shall be stamped on one end of the axle no later than January 18, 2005.