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The Severn Tunnel ((ウェールズ語:Twnnel Hafren)) is a railway tunnel in the United Kingdom, linking South Gloucestershire in the west of England to Monmouthshire in south Wales under the estuary of the River Severn. The tunnel was built by the Great Western Railway (GWR) between 1873 and 1886. It is 4 miles 624 yd (7,008 m) long, although only 2¼ miles (3.62 km) of the tunnel are under the river. For well over 100 years it was the longest mainline railway tunnel within the UK, until the two major High Speed 1 tunnels (London East and West) were opened in 2007 as part of the Channel Tunnel Rail Link. It takes approximately 3 minutes 40 seconds to travel through the tunnel. == General == The tunnel is a critical part of the trunk railway line between southern England and South Wales, and carries an intensive passenger train service, and significant freight traffic. The whole length of the tunnel is a single signal section, limiting the headway of successive trains. The steep gradients (1 in 90 and 1 in 100) make the working of heavy freight trains difficult. There is a continuous drainage culvert between the tracks to lead ground water away to the lowest point of the tunnel, under Sudbrook Pumping Station, where it is pumped to surface. The hazard of ignited petroleum running into the culvert in the event of derailment of a tank wagon means special arrangements have to be made to prevent occupation of the tunnel by passenger trains while hazardous liquid loads are being worked through. Special evacuation arrangements are in place to enable the escape of passengers and staff in the event of serious accident in the tunnel. There is restricted personnel access to the tunnel at Sudbrook Pumping Station, where an iron ladder descends in the shaft of the water pumping main; the ventilation air is pumped in at this point also. The GWR original ventilation arrangement was to extract air at Sudbrook, but the aggressive exhaust gases from the then steam train operation led to premature corrosion of the fan mechanism, and when the Cornish engines were replaced in the 1960s the draughting was reversed so that atmospheric air is pumped in to the tunnel exhausting at the tunnel mouths. On average, around 50 million litres of water per day of fresh (spring) water are pumped from the tunnel and released into the River Severn. 〔2013 figure, http://www.networkrail.co.uk/browse%20documents/projects/great%20western/electrification%20environmental%20statements/wales/gwml%20electrification%20-%20severn%20tunnel%20cardiff%20-%20env.%20statement%20vol%202%20appendix%20e%20-%20flood%20consequences%20assessment%20study.pdf〕 In the event of non-availability of the tunnel, trains are diverted via Gloucester. The especially difficult conditions for infrastructure maintenance in the tunnel, as well as the physical condition of the tunnel structure, require a higher than normal degree of maintenance attention. Access and personal safety difficulties mean that significant work tasks can only be done during temporary line closure. Some 200 trains per day use the tunnel. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Severn Tunnel」の詳細全文を読む スポンサード リンク
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