This part was overlooked as I had forgotten I had this data available from my old Google Earth maps. When NZ Rail Maps started 10 years ago the maps were first drawn with Google Earth. This had many limitations so that is why I switched to Qgis after three years. Now with the combination of the Qgis authored maps and Linz aerial photography I can have the best of both worlds.
In 1937 surveys of the Nelson line beyond Longford were carried out. Two options were considered, basically the opposite sides of the Buller River. The true left bank option would have basically followed the same route as constructed to Mangles River, then carried on through Murchison and further west. The true right bank option would have passed Murchison on the far side of the river and a bridge would have been needed for the township to access the railway station. Both surveys finished up at Ariki Falls, which is a little west of Ariki Junction, where Highway 6 to Westport and Murchison joins Highway 65 to Springs Junction. SH65 is currently experiencing record traffic volumes due to the diversion of road freight off SH1 following the Rotherham Earthquake.
The Buller Gorge is rugged terrain and the railway from Murchison to Inangahua Junction, as was always proposed, would have been an expensive project to complete, and was thrown in doubt following the 1929 Murchison earthquake. But mostly it was the slow speed of extending the line south of Belgrove that finished it because the 1930s when construction finally stopped was a time when a lot of railway lines around the country stopped being built, or in some cases partially closed. The first section of the Nelson line was opened in 1876 to Foxhill (34 km) and Belgrove (36 km) was reached in 1881. However there was then a delay to try to work out the correct route to take the line south with the first option being to head towards Tophouse as this was supposedly easier than the route eventually taken through the Spooner range. The indecision over the route south of Belgrove slowed things down and it was not until the 1890 that the route through the Spooner range with the tunnel was started by the Midland Railway Company which had the contract to build parts of the West Coast network and link it up to the Nelson line. Belgrove actually had two railway stations with the differing alignments, that (along with the 1883 route) will be covered in more detail with the series part dealing with that section of the railway. It took 10 years to push the railway through to Motupiko partly because the Midland company went bust in 1895, and another six to get to Tapawera, then six more to Glenhope. So from Belgrove to Glenhope which was 61 km of track, it took 31 years to build. Then from Glenhope to Gowanbridge, only 13 km, took another 17 years. And after the work stopped south/west of Gowanbridge in 1931 it was not until 1949, another 18 years, that the government proposed restarting construction of the line.
Here are the two routes at 1:50000 scale (Topo50 scale) as the original was not detailed enough for a larger scale to be of any use.