Severn Tunnel
The Severn Tunnel (Welsh: 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. Its completion represented the crowning achievement of the great Victorian civil engineer Sir John Hawkshaw. It is 4 miles 624 yd (7,008 m) long, although only 2 1⁄4 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 was also the longest underwater tunnel in the world until 1987. It takes approximately 3 minutes 40 seconds to travel through the tunnel.
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 the 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. 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 exhaust gases from steam train operation led to premature corrosion of the fan mechanism. When the Cornish pumping 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.
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