From Graces Guide
7,662 Yards. Severn Tunnel - Great Western Railway
The Severn Tunnel 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 2.25 miles (3.62 km) of the tunnel are under the river.
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.
Prior to the building of the tunnel, the railway journey between the Bristol area and South Wales involved a ferry journey between New Passage and Portskewett or a long detour via Gloucester. The rail journey time could be hugely shortened by construction of a tunnel; work began in March 1873 and proceeded gradually through the 1870s.
Thomas Andrew Walker, the engineer for the work, notes in his book, that the GWR had expected the critical part of the work to be the tunnelling under the deep-water channel of the Shoots. However, the real difficulties began in October 1879, when only 130 yards (119 m) remained separating the main tunnel heading being driven from the Welsh side and the shorter Gloucestershire heading, the workings were inundated. The incoming water was fresh, not from the Severn but from the Welsh side, and the source became known as "The Great Spring".
Thomas A. Walker was the contractor entrusted by the chief GWR engineer John Hawkshaw with rescuing and completing the tunnel after the 1879 flooding. Holding the Great Spring in check required the installation of greatly increased pumping facilities, and a diver had to be sent down a shaft and 300 m along the tunnel heading to close a watertight door in the workings and seal off the waters.
This troublesome task was finally achieved in November 1880 by lead diver Alexander Lambert using Henry Fluess' new self contained breathing apparatus, but work in the area of the Great Spring was unable to continue until January 1881 when the Great Spring was temporarily sealed off.
Work was later disrupted in 1883 by further flooding from the Great Spring, and again Lambert managed to save the day. Additional mishaps afflicting the workings included a large tidal wave and a breakthrough of the bed of a pool (the "Salmon Pool") on the English side.
In the intervening period the Severn Railway Bridge from Sharpness to Lydney was opened.
The tunnel was completed during 1885 and a goods train passed through it on 9 January 1886, but regular services had to wait until the pumping systems were complete.
The tunnel opened to goods trains in September and to passenger traffic in December 1886, nearly 14 years after work had started. Fixed Cornish beam engines pumped out the Great Spring and other sources of water until the 1960s, when they were replaced by electrically powered pumps.
In 1924, the Great Western Railway started a service to transport cars on rail trucks through the tunnel between Pilning and Severn Tunnel Junction. The service was an alternative to the Aust Ferry, which had an erratic timetable determined by the tides, or the long road journey via Gloucester. The service continued after the war, but was made redundant by the opening of the Severn Road Bridge in 1966.
During World War II, a Great Western Railway passenger train was pursued by a German aircraft along the main line to Wales. Reaching speeds estimated at 90 mph, well above the wartime restrictions in place, the train successfully escaped into the tunnel and stopped beneath the river until the driver judged that the danger had passed. The train was struck by several bullets during the chase but there were no serious injuries
Sources of Information
-  Wikipedia