Railway electrification in New Zealand consists of three separate electric systems, all on the North Island. Electrification was initially adopted by the New Zealand Railways for long tunnels; the Otira Tunnel, the Lyttelton Rail Tunnel and the two Tawa Tunnels of the Tawa Flat Deviation. Electrification of Wellington suburban services started with the Johnsonville Line and Kapiti Line out of Wellington from the 1930s. Auckland suburban services were electrified in 2014–2015. Electrification of long-distance services on the North Island Main Trunk (NIMT) dates from 1986. New long tunnels, for example the Rimutaka Tunnel and the Kaimai Tunnel, were operated by diesels, and the Otira and Lyttelton Tunnels have converted to diesel operation.
From 1908 to 1953, there was an electrified mine railway from the Stockton mine on the West Coast of the South Island.
Earlier NZR electrified routes from 1923 to the 1940s operated at , but the NIMT (1986) and Auckland suburban services (from 2014) use ; all with overhead catenary supply. The use of 16 kV 16.7 Hz AC for the NIMT was proposed in 1950.
History
Stockton mine railway
The Stockton mine railway was, in 1908, New Zealand's first electric railway. It carried coal from the Westport-Stockton Coal Companies mine to the NZR railhead at Ngakawau on the West Coast of the South Island from 1908 to 1953, when it was replaced by an aerial cableway. The line was long, with in two long tunnels. The system used 275 V DC from a low overhead line via trolley poles, and gauge track. The seven locos were of low built "mine" type.
Otira and Lyttelton Tunnels
The first NZR line to be electrified in New Zealand was the Otira Tunnel in 1923. This long tunnel () with a steep gradient (1 in 33) could not have been worked by steam. In 1916, there was consideration of electrification of the entire section with steep grades from Jackson (West Coast) to Springfield (Canterbury), but in 1923, just the Otira-Arthur's Pass section () was electrified.
In 1929, the Lyttelton Line through the Lyttelton Rail Tunnel was electrified. This tunnel (opened in 1867) was short and had a lesser gradient than the Otira (0.3%) but replacement of steam operation was desirable. Electrification of this and other lines was being studied by the Public Works Department as early as 1911.
The Merz & McLellan Report
The 1925 report by the English consulting firm of Merz & McLellan was commissioned by the Minister of Railways Gordon Coates to investigate electrification of suburban services in the four main centres of Auckland, Wellington, Christchurch and Dunedin. For Christchurch it recommended electrification of the Lyttelton Line but not the main lines north and south. Now only Auckland and Wellington have suburban passenger services. The firm's partner Charles Merz of Newcastle upon Tyne had reported on Melbourne suburban electrification in 1908 and 1912.
Wellington electrification
Several Wellington lines were then electrified: in 1938, the Johnsonville Line; and from 1940, the Kapiti Line section of the NIMT north of Wellington to Paekakariki through the two Tawa Tunnels which were part of the Tawa Flat Deviation. This line also had steep gradients (1 in 57) on the bank from Paremata up to Pukerua Bay.
In February 1946, it was decided to electrify the remaining Wellington suburban lines to the Hutt Valley as there was a shortage of coal for locomotives, and also to replace commuter steam trains with EMUs, as the Hutt Valley was now largely residential with new state housing replacing market gardens. When the extra EMUs arrived in 1949, they were initially used for the Kapiti Line to Paekakariki while the Hutt Valley lines to Upper Hutt and Melling were electrified. So in the Hutt Valley, EMUs were supplemented in peak periods by older carriages hauled by electric locomotives until they were replaced by the Ganz-Mavag EMUs from 1986.
The Johnsonville and Melling lines were short branch lines which were originally part of the main line (NIMT) north and the Hutt Valley line to the Wairarapa, until the lines were replaced by deviations. The sections of these lines that were retained were mainly used for suburban commuter services, with initially some stock traffic to Johnsonville (later to Raroa).