|Otira Tunnel during construction, ca 1910|
|Location||Southern Alps, South Island, New Zealand|
|Coordinates||North (West coast) portal:
East (Canterbury) portal:
|Start||Otira, West Coast|
|End||Arthur's Pass Canterbury|
|Opened||4 August 1923|
|Operator||KiwiRail, Tranz Scenic|
|Character||Single bore rail tunnel|
|Line length||8566 m|
|Track gauge||1067 mm (3' 6")|
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 over 8.5 kilometres (5.3 mi). The gradient is mainly 1 in 33, and the Otira end of the tunnel is over 250 m (820 ft) lower than the Arthur's Pass end. Construction started in 1907 and it opened on 4 August 1923; at the time of its construction, it was one of the longest tunnels in the world.
Because of 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 with steam engines. Thus, the tunnel was electrified with a 1500V DC overhead system. A small coal-fired power station was built near Otira to provide electricity until 1941, when it was replaced by a connection to the national grid. The locomotives used were the EO class, then from 1968 the EA class.
Because of the increasing age of the electrification and the availability of upgraded DX Class diesel locomotives, the electrification was decommissioned in 1997 and the equipment removed. This marked the end of electrification in the South Island.
To overcome the fume problem, a combination of a door and fans is used, similar to that used in the Cascade Tunnel in the USA, which was also once electrified. 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. Because of the fumes, the TranzAlpine's observation cars are closed for the trip through the tunnel.