Railway wagon axles are made by heating rods of iron embedded in charcoal powder. The process is known as

Case hardening: The process of hardening the surface of wrought iron by depositing a surface layer of steel on it is called case-hardening it is done by heating wrought iron in contact with potassium ferrocyanide. Alternatively, case hardening can also be done by heating wrought iron with charcoal and then plunning it a suitable oil.