The Seddonville Branch, later truncated as the Ngakawau Branch, was a branch line railway in the West Coast region of New Zealand's South Island. Construction began in 1874 and it reached its terminus at the Mokihinui Mine just beyond Seddonville in 1895. In 1981 it was closed past Ngakawau and effectively became an extension of the Stillwater - Westport Line, since formalised as the Stillwater Ngakawau Line.
The branch was built for transporting coal from mines to the harbour at Westport. Unlike most other railways of the era, there was no expectation that it would open up country for settlement and farming, as the terrain was mountainous and not suited to settlement of significant size. Coal had been discovered on the north bank of the Waimangaroa River on 25 March 1873, and on 3 March 1874 tenders were called for the construction of a railway to link Waimangaroa with Westport. Construction began soon after, and the first section opened to Fairdown on 31 December 1875. Waimangaroa was reached on 5 August 1876 and regular services began operating. The extension to Ngakawau opened on 12 September 1877, bringing the length to 29.98 kilometres. 
The impact of the Long Depression limited government funds available for railway construction, and no extension of the line occurred for over a decade. By the end of the 1880s, the economic position was improving and work commenced on extending the line to Seddonville. On 8 August 1893 it opened to Mokihinui, and on 23 February 1895 the New Zealand Railways Department acquired a 6.2-kilometre private line from Mokihinui through Seddonville to the Mokihinui Mine, run by the Mokihinui Coal Company. Some of the funds for the construction of the Ngakawau-Mokihinui section and the purchase of the Mokihinui line were provided by the Westport Harbour Board.
The following stations are or were on the branch (in brackets is the distance from Westport):
- Sergeants Hill (5 km)
- Fairdown (10 km)
- Waimangaroa (15 km) - junction with the Conns Creek Branch, 1877-1967.
- Birchfield (20 km)
- Granity (27 km)
- Ngakawau (30 km)
- Hector (31 km)
- Nikau (39 km)
- Summerlea (40 km)
- Mokihinui (42 km)
- Seddonville (45 km)
- Mokihinui Mine (48 km)
There were never passenger trains due to the low population: the Railways Department operated mixed trains, goods trains with passenger carriages attached. For a period they ran to Mokihinui Mine, but on 12 June 1933 they were cancelled beyond Seddonville. Services to Seddonville lasted another 13 years, and on 14 October 1946 the branch closed to passengers. Since then, the only passenger trains have been infrequent enthusiasts' excursions.
In the branch's first half century, freight was not confined to coal. However, as road transport became more prevalent, local businesses abandoned rail cartage and coal was virtually the only freight carried by the late 1930s. Coal tonnages were declining by this stage: in 1940, the branch was carrying just over half its pre-World War I peak of 800,000 tons. Nonetheless, coal traffic was more than sufficient to keep the branch in service. An early 1967 timetable had one train to Seddonville and the Mokihinui Mine and two to Ngakawau on weekdays, with shuttles from the Conns Creek Branch that diverged at Waimangaroa. Traffic from the Conns Creek Branch was declining and it closed later that year.
The three-kilometre section beyond Seddonville to Mokihinui Mine closed on 10 February 1974 after the mine closed. Low demand for Buller region coal, decreased output and a decline in coastal shipping to Westport meant that the remainder carried reduced tonnages. Closure beyond Ngakawau was proposed in 1976, and although it operated a few more years, maintenance costs were increasingly higher than revenue and the branch beyond Ngakawau closed on 3 May 1981.
In the 1980s, traffic rose significantly despite closure beyond Ngakawau and trains ran across the South Island via the Midland Line to the deepwater harbour at Lyttelton rather than to Westport. In 1981, only 117,000 tonnes of coal were carried to Lyttelton; by 1989, this had risen to 600,000 tonnes. Due to the boom in traffic, bogie coal wagons were built to replace ageing four-wheeled stock of much lower capacity. In mid-June 2007, traffic was sufficient to justify five trains daily to Lyttelton.
In September 1875, three C class 0-4-2T tank locomotives were shipped to Westport in readiness for the opening to Fairdown. It soon became apparent that greater motive power was required, and in 1898 four WB class tanks were delivered. Three decades later they were followed by WW class tank locomotives, the first of which arrived in 1929, which were the mainstay until dieselisation. However, the WWs were limited on the steep 1 in 33 gradient beyond Seddonville to the Mokihinui Mine, capable of hauling only 180 tons. Occasionally other steam locomotives operated, including UC class tender locomotives.
In late 1967 dieselisation began in earnest with the arrival of DSC class and DJ class locomotivess. A year later, the WW class had been largely replaced by DSCs beyond Waimangaroa. By mid-1969 steam power had ended, and DJ locomotives, joined by the DC class in the 1980s, became the predominant motive power. With the de-electrification of the Otira Tunnel on the Midland Line in the latter half of the 1990s, motive power changed to powerful DX class locomotives modified to operate through the tunnel.
Some remnants of the branch beyond Ngakawau remain. Much of the formation remains visible, including embankments, cuttings and culverts. Part of the formation near Seddonville has been converted into the Chasm Creek Walkway, uses two bridges and one tunnel. The platform edge of Seddonville station remains, but the line to the Mokihinui Mine is now largely inaccessible as it is not near any roads.
At the end of their working lives, some locomotives and rolling stock were used to protect the branch from river erosion. Between 1958 and 1960, WB class tank locomotives 292 and 299 were dumped on the bank of the Mokihinui River to ensure the stability of the formation between Seddonville and Mokihinui Mine. In 1989 the Baldwin Steam Trust recovered the locomotives with the aim of restoring them to full operational condition.
- Hermann, Bruce J; South Island Branch Lines pp 21-23 (1997, New Zealand Railway & Locomotive Society, Wellington) ISBN 0-908573-70-7
- Leitch, David (1972). Railways of New Zealand. Melbourne: Lothian Publishing. pp. p. 41. ISBN 0-85091-118-4.
- Leitch, David; Brian Scott (1998). Exploring New Zealand's Ghost Railways. Wellington: Grantham House. pp. p. 52.
- Churchman, Geoffrey B.; Tony Hurst (1991). The Railways of New Zealand: A Journey Through History. Auckland: HarperCollins. pp. p. 192. ISBN 1-86950-015-6.
- Patrick Dunford, "Seddonville Branch", accessed 23 June 2007.
- Leitch and Scott, 53.
- Hurst, Tony (1995). Farewell to Steam: Four Decades of Change on New Zealand Railways. Auckland: HarperCollins. pp. p. 58. ISBN 1-86950-187-X.
- New Zealand Train Timetable Guide, accessed 20 June 2007, last updated of guide on 7 June 2007.
- Hermann, Bruce (1997). South Island Branch Lines: Pictorial Memories. Wellington: New Zealand Railway and Locomotive Society. pp. p. 21. ISBN 0-908573-70-7.
- Churchman and Hurst, 193.
- Churchman and Hurst, 190.
- Hermann, 22.
- Leitch and Scott, 54.
- Baldwin Steam Trust, "Disposal of Baldwin locomotives Wb 292 and 299", accessed 24 June 2007.
- Baldwin Steam Trust, "Salvage Scheme", accessed 23 June 2007.