49 CFR 215.105: Defective axle.

A railroad may not place or continue in service a car, if—

(a) An axle on the car has a crack or is broken;

(b) An axle on the car has a gouge in the surface that is—

(1) Between the wheel seats; and

(2) More than one-eighth inch in depth;

(c) An axle on the car, used in conjunction with a plain bearing, has an end collar that is broken or cracked;

(d) A journal on the car shows evidence of overheating, as evidenced by a pronounced blue black discoloration; or

(e) The surface of the plain bearing journal on the axle, or the fillet on the axle, has—

(1) A ridge;

(2) A depression;

(3) A circumferential score;

(4) Corrugation;

(5) A scratch;

(6) A continuous streak;

(7) Pitting;

(8) Rust; or

(9) Etching.