Thursday, 8 April 2010

South Island Main Trunk, Dunedin-Palmerston, Part 1: Introduction

Over the recent Easter weekend I was in Dunedin and took the opportunity to travel on the Taieri Gorge Railway’s Seasider tourist service which operates twice weekly through the high season and occasionally in the low season between Dunedin and Palmerston on the South Island Main Trunk. Although I have visited Dunedin a number of times, I have travelled on this particular section by train only three times, and all of them were specials: two NZRLS convention feeder services in October 1987 and an OETT excursion to Hampden and Makareao (via the former branchline) in January 1988. The furthest south I ever got on the Southerner when it was running was Palmerston itself.  This trip was the first opportunity I have had to make a comprehensive photographic/geographic record of the section, which can be viewed here in its entirety, and of which highlights are included in a later part of this multi part blog post. This first article details mainly the history of the route. The second part will detail the major changes on maps, and a subsequent post will cover the Seasider trip itself as I rode on it.
The South Island Main Trunk (Main South Line) was constructed as separate works from the three main provincial centres. At Christchurch, work began with the Ferrymead Railway in 1863 between Heathcote and the city, later on being bypassed by the section through to Lyttelton via the tunnel, and south/east of the city the line crept down the coast in stages, reaching Selwyn in 1867, Rakaia in 1873, Ashburton in 1874, Ealing in 1875, Hillgrove in 1876, and finally Waikouaiti in 1878. However the formal driving of the last spike in June 1877 took place at Goodwood, somewhat north of Waikouaiti. The work at Dunedin started in 1873 with the opening north to Port Chalmers. The section between Sawyers Bay and the port became a separate branch line and the remainder the SIMT which advanced up the coast to Waitati in 1877, reaching Waikouaiti from the south in 1878. South of Dunedin and north of Invercargill, construction proceeded simultaneously with both projects opening their first sections in 1874, and meeting at Balclutha in 1879.
As was typical of the era, even a major route as the Main Trunk was built with typically steep gradients and sharp curves in heavy country, and as the traffic developed it became necessary to effect a continuous program of improvements wherever possible. Heading south from Christchurch, the route is essentially flat and straight right through to Oamaru where the hills begin in earnest. On my last trip to Dunedin I detailed the large scale improvements between Dunedin and Mosgiel in the city suburban section that were found necessary after thirty years of operation. The same challenge has faced the railway operations northward from Dunedin to Merton, which is a particularly demanding operation beset with incessant steep gradients, small radius curves and unstable ground. Due to the hilly country through which the railway passes, major improvements in this area would only be possible by constructing a large scale deviation with extensive tunnelling. Between 1908 and 1948 the line was straightened, levelled and doubled within the suburban area from Dunedin to Sawyers Bay. North of there, various proposals have surfaced from time to time to construct a long tunnel bypassing the most difficult sections of the route, but this has never eventuated and is now an unlikely proposition. Travel over this 45 km section remains, therefore, a slow ride with the effective ruling speed the 45 km/h limit imposed on numerous sharp curves, and it explains for the most part why the Seasider takes 90 scheduled minutes to cover a 63 km journey. The 45 km is effectively split itself into two sections from Sawyers Bay to Waitati and Evansdale to Merton, these paralleling the two sections of State Highway 1 which are respectively Dunedin-Waitati via Leith Saddle and Waitati-Waikouaiti via the Kilmog Hill.
The unstable ground is particularly prevalent both for road and rail in the Waitati-Waikouaiti section (Kilmog) where the prevailing geology is a mixture of sandstone and mudstone, the movement of the latter at the Puketeraki Tunnel necessitating its abandonment and bypass by a deviation on a fairly sharp curve that was constructed around 1934-36. The Kilmog highway, further up the same hill, is also rough in a few places due to the movement of the land and regularly has to be realigned. Photography of the coastline in the area shows that the cliff faces look like porridge at their foot where they meet the sea, which is consistent with the characteristics observed in other major unstable areas such as parts of the Main North Line around Oaro where I have also travelled by train and observed the geology, and notorious places where I haven’t been such as the original Mangaweka section of the North Island Main Trunk. The curve around the old tunnel is quite sharp, the radius could be as small as 100 metres which would impose a fairly severe speed limit and we went through the area both ways at, according to the GPS, 30 km/h which is probably commensurate with such a curve size although the slowness could also be due to other reasons such as unpredictable movement in the trackbed or landslip risk that would be expected in an unstable area. Using two measurements with the aid of Google Earth and GPS data I estimate that the curve’s actual radius is around 120-130 metres, most probably sharper than anything else on the route. Curves of such a small radius are uncommon on any main line route, but there are a lot at 140-150 metres between Sawyers Bay and Merton which have never been able to be improved because of the topography of the area. There have also been land movement problems observed in the Mihiwaka Tunnel (between Sawyers Bay and Waitati) requiring lining reinforcement, and at the Cliffs Tunnel in the same section of track, there is a suggestion which I have not yet confirmed, that the railway originally went around the side of the tunnel on the cliff face and was put through the tunnel later because the cliffside route was considered unsafe, possibly due to slipping. North of Merton although there is still the challenge of gradients imposed by the undulating terrain through to Oamaru, there have been fewer engineering challenges over the past 133 years and some improvements such as curve easing have been effected and of course the renewal of bridges, level crossing elimination and stable high speed concrete sleepered track make this section a great deal more suited to its current role as the South Island Main Trunk railway up the east coast of the island.
Well that wraps up this introduction to the route. The next part will show a route map and describe major features and changes that have been made in the section. It will be a few days until I get to writing about the Seasider itself, as I have to finish editing the photo album before then. You can expect to see a few lesser quality or duplicated images removed before the captions are added to the rest. They have all been geotagged using the GPS tracks that I obtained from my Seasider trip. Some of the photos will be used to illustrate the Seasider part of this series.