A part of the superheater in a steam locomotive, superheater elements are the long pieces of pipe, folded back on themselves in a tight U, through which the steam to be superheated passes. The superheater elements are inserted inside enlarged boiler tubes, called flues, and therefore are directly exposed to hot combustion gases. Superheater elements are a commonly replaced part of a locomotive. They are fitted into the superheater header at the smokebox end of the locomotive.