Otira Tunnel
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The Otira tunnel is a major railway tunnel on the Midland Line in the South Island of New Zealand between Otira and Arthur's Pass. It was constructed between 1907 and its opening on August 4, 1923, and it runs under the Southern Alps from Arthur's Pass to Otira - a length of a little over 8500 metres. The gradient is close to 1 in 30, and the Otira end of the tunnel is over 250 metres lower than the Arthur's Pass end. At the time of its construction, it was one of the longest rail tunnels in the world.
Due to its length and gradient, gases such as carbon dioxide and carbon monoxide could easily build up, potentially making the tunnel both unhealthy for the train's occupants and unworkable for its engines. Thus, the tunnel was electrified with a 1500 V DC overhead system. A small hydroelectric power station was built near Otira to provide electricity. Due to the increasing age of the electrification and the availability of upgraded DX Class diesel locomotives the electrification was closed and the equipment removed. This marked the end of electrification in the South Island.
To overcome the fumes problem, a combination of a door and fans is used. When a train enters the tunnel from the Otira end, the door is shut over the entrance, and a large fan extracts the fumes behind the train. Once the fumes have been extracted, the door is reopened.
