49 C.F.R. § 215.105 Defective axle.
Title 49 - Transportation
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.
Title 49: Transportation
PART 215—RAILROAD FREIGHT CAR SAFETY STANDARDS
Subpart B—Freight Car Components
Suspension System
§ 215.105 Defective axle.