As we all know there are various locations around the country where, to save on the construction costs of bridges, railway and road bridges have been combined on the same substructure (foundations and piers). The earliest bridges of this type also had a combined single level deck which was essentially a road deck with rails added to the top. The best known example was the Rakaia River bridge over 1.6 km long. New road and rail structures replaced the old wooden bridge there in 1940. Although by the time of writing this type of bridge is about to disappear completely on the KRL network, two bridges of this type are still in use for both purposes on the Taieri Gorge Railway.
The next design iteration produced bridges with separate decks on two levels. All examples of this in NZ have been with a road deck at the lower level and a train deck at the upper level. The three bridges of this type are at Awatere on the Main North Line (rail remains on the upper level with the lower deck removed), Karangahake on the route of the former East Coast Main Trunk Railway route that was bypassed in 1978 (the lower deck is still a road and the upper deck now carries a rail trail), and Okahukura (the lower deck is still a road and the upper deck now carries track for the Forgotten World Adventures rail carts operation).
The third design has been constructing parallel decks on the same single level. There are three bridges of this type in New Zealand. The bridge at Westshore (Napier) was started somewhat earlier than the Inangahua one; both were completed in the 1920s. At Westshore the bridge carries the disused PNGL alongside a closed road. At Inangahua the road was completed first, the rail tracks had to wait until the 1930s to be completed. The third bridge of this type completed in 2006 is at Arahura on the Hokitika Line. There, a new two lane traffic bridge and rail bridge side by side replaced a single deck wooden combined bridge.
The Inangahua bridge was significantly damaged in the Murchison Earthquake of 1929 when the piers were badly cracked. The repair consisted of casting larger diameter piers around the damaged lower portions of the originals. Further damage was experienced in 1968 due to the Inangahua Earthquake. However unlike the single deck combined structure at Landing to the south, which was patched up until it could be replaced 14 years later, the Inangahua bridge was able to be successfully repaired. It still operates today for both forms of traffic and there are no known plans to replace it on either level.
A couple of archival photos of the Inangahua bridge after the 1929 Murchison earthquake showing the damaged original piers. The road deck was on the bridge, but the rails had not been added at this time. The bridge girders for the railway line were built by Andersons Engineering in Christchurch and railed to Inangahua, where the PWD had a narrow gauge tramway that they used to bring the parts up to the site. Probably they would have strung a cableway across to help move the spans into place. This was completed in the 1930s.
Taken at the north end of the bridge we have the road and rail decks side by side.
Looking down the side of the road deck from the south end we can see the larger diameter of the lower piles as they were recast to repair the earthquake damage. As can be seen in both this photo and the archive one the road deck is essentially cantilevered over the side of the bridge piers with extensions to the upper sections. Although there could be many reasons for this, one possibility is that a narrower single deck layout was originally planned for the site. The construction of this bridge was started in 1923 only 9 years after probably the last single deck structure was completed at Parnassus on the Main North Line and the parallel deck structure may well have been a late design change.