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. The Midland line is a famous 212 km section of railway between Christchurch (Rolleston) and Greymouth in New Zealand. ... The South Island The South Island is one of the two major islands of New Zealand, the other being the North Island. ... Arthurs Pass Arthurs Pass is an alpine crossing of the Southern Alps of the South Island of New Zealand. ... 1907 (MCMVII) was a common year starting on Tuesday (see link for calendar) of the Gregorian calendar (or a common year starting on Wednesday of the 13-day-slower Julian calendar). ... August 4 is the 216th day of the year in the Gregorian Calendar (217th in leap years), with 149 days remaining. ... 1923 (MCMXXIII) was a common year starting on Monday (link will take you to calendar). ... The Southern Alps is a mountain range which runs along the western side of the South Island of New Zealand. ...

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 equiment removed. This marked the end of electrification in the South Island. Carbon dioxide is an atmospheric gas comprised of one carbon and two oxygen atoms. ... Carbon monoxide, chemical formula CO, is a colorless, odorless, tasteless, flammable and highly toxic gas. ... Overhead wire in Coventry, England Overhead wire and its suspension system in Bridgeport, Connecticut, USA A railway electrification system is a way of supplying electric power to electric locomotives and multiple units. ... Josephson junction array chip developed by NIST as a standard volt. ... Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ... Hydroelectricity is a form of hydropower used to produce electricity. ... The General Electric DX class, consists of forty-nine locomotives built by General Electric of the United States between 1972 and 1975 for the then New Zealand Railways Department. ...

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.