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 August 4, 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 1500 V DC overhead system. A small coal-fired power station was built near Otira to provide electricity. The locomotives used were the EO class, then from 1968 the EA class.