Ōtira rail tunnel opened

4 August 1923

The 8.5-km Ōtira tunnel, which pierced the Southern Alps and linked Christchurch with Greymouth, was formally opened by Prime Minister William Massey. At the time it was the longest tunnel in the southern hemisphere, the longest in the British Empire, and the sixth-longest in the world.

Work on the transalpine (or ‘Midland’) line had begun in January 1887. This ambitious private project, launched by the New Zealand Midland Railway Company, aimed to connect the West Coast with not just Canterbury but also Nelson. But progress was painfully slow and in 1895 the project was taken over by the government’s Public Works Department (PWD), triggering legal disputes and further delays. The West Coast section reached Ōtira by 1900 and tenders for a long tunnel through the Alps to Arthurs Pass, 737 m above sea level, were called in 1907.

Contractor J.H. McLean & Sons began work the following year, but the project was plagued by engineering problems, extreme weather conditions and labour shortages, which eventually led to the company’s collapse. In 1912 the PWD took over but work slowed during the First World War. When the two ends of the tunnel were joined in 1918 the surveyors’ centre lines were found to vary by less than 30 mm, an impressive level of accuracy for the era. The line was finally opened for traffic in 1923 – 36 years after work had first begun on a transalpine link.

Due to its length and continuous steep gradient (1 in 33), it was decided to electrify the tunnel using a 1500V DC overhead system. Electric power was supplied by a small coal-fired station near Ōtira and later by hydroelectric supply. The tunnel section of the line was initially served by five English Electric Eo-class locomotives, which were replaced in 1968 by Toshiba Ea-class (later called EO-class) locos. The tunnel’s electric system was decommissioned in 1997 in favour of diesel power, with the locomotives' fumes being dealt with by a system of doors and exhaust fans.

From the 1920s to the 1960s popular Sunday excursions were run from Christchurch to Arthur’s Pass and Ōtira. Today, the tunnel is heavily used by freight trains carrying West Coast coal to Lyttelton for export, as well as by the TranzAlpine tourist train.

Image: Ōtira tunnel opening

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