While all structures erected by the NZASM along their lines were built in situ, they were in fact a 'kit of parts' of predetermined size and design, each appropriate to the circumstances. It must have been apparent that, although often dry in winter, when summer rains came to the highlands where most of the rail was located, flash flooding might easily inundate or damage rail infrastructure. Although precautions were take, there was still severe damage by flooding along the Barberton Branch line during construction, even before the rail was commissioned, and the necessary steps were taken to heighten bridges during repairs.
When one travels along the trajectories of the old NZASM lines one will often notice stone culverts, many still in place and thundered over by the freight and passnger trains that traverse these lines even today, although tracks have been raised, concrete added, heavier rail profiles laid and trajectories straightened. Where lines have been doubled, tripled or even quadrupled often the original stone masonry remains and subsumed into the larger requisite concrete structures.
Flat lintolled culverts generally have small stone piers and are constructed with one, two or three drainage channels. Initially they were bridged with steel rail lintols encased in mass concrete.
Arched culverts are constructed of rough ashlar with vousoirs and keystones. Often the stones for the vaulting is of softer sandston while the rest of the stone masonary is of harder local stone - granite, dolerite or modderklip (a hard metamorphosed secondary rock).
Viaducts are often constructed of larger arched structures with one or more causeways, either equi-spanned or symetrical smaller spans about a larger central vault.