Otira Tunnel

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Otira Tunnel
Info
Line	Midland Line
Location	Southern Alps, South Island, New Zealand
Start	Otira, West Coast
End	Arthur's Pass Canterbury
Operation
Opened	August 4, 1923
Owner	ONTRACK
Operator(s)	Toll NZ, Tranz Scenic
Character	Single bore rail tunnel
Technical
Line length	8.5km
Gauge	1067mm

The Otira Tunnel is a railway tunnel on the Midland Line in the South Island of New Zealand between Otira and Arthur's Pass. It runs under the Southern Alps from Arthur's Pass to Otira - a length of a little over 8500 metres. The gradient is mainly 1 in 33, and the Otira end of the tunnel is over 250 metres lower than the Arthur's Pass end. Construction started in 1907 and it opened on August 4, 1923: at the time of its construction, it was one of the longest 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 in 1997 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. After a train enters the tunnel from the Otira end the door closes off the entrance, and a large fan extracts the fumes behind the train. Once the fumes have been extracted, the door is reopened. Due to the fumes, the TranzAlpine's observation cars are closed for the trip through the tunnel.^[1]

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