Severn Tunnel

The Severn Tunnel forms a critical part of the trunk railway line between southern England and South Wales, and carries an intensive passenger train service as well as significant levels of goods traffic. As of 2012, an average of 200 trains per day use the tunnel. The whole length of the tunnel is controlled as a single signal section, which has the consequence of limiting the headway of successive trains. 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 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 performed during temporary line closure, during which trains are normally diverted via . British NGO Engineering Timelines state that the tunnel would be full of water within 26 minutes if the pumps were switched off and backup measures failed, while Network Rail has also observed that the corrosive atmosphere of the tunnel, produced from a combination of moisture and diesel fumes from passing trains, results in so much corrosion that the steel rails need to be replaced every six years. ==History==

Construction . Prior to the building of the tunnel, the railway journey between the Bristol area and South Wales involved a ferry journey between and or a long detour via . Officials within the Great Western Railway (GWR) soon realised that the rail journey time between the two locations could be significantly shortened by construction of a tunnel directly underneath the River Severn. On 18 March 1873, construction activity commenced using labourers employed directly by the GWR; this initial work was focused on the sinking of a shaft, possessing a diameter of at Sudbrook and a smaller drainage heading near the Pennant Measures. It had been originally assumed that the continuous brickwork lining of the tunnel would withstand the groundwater pressure, thus the drainage sluice valve on the side heading was closed and all but one of the pumps were taken from the site. Operations At the newly built station, the GWR built a major marshalling yard, which: distributed east and north, sending coal from the South Wales Valleys towards London and the Midlands; created mainline and localised mixed-traffic freight from goods shipped in from the Midlands, the Southwest and along the Thames Valley, both westwards into Wales and vice versa. enters the Severn Tunnel in 1997 from the Welsh side under Caldicot. Due to the access gradients, throughout the steam era, assistance was required for the passage of all heavy trains through the Severn Tunnel, which entailed (eastwards, from ): of 1-in-90 down to the middle of the tunnel; a further at 1-in-100 up to ; a short level then more at 1-in-100 to . A number of fixed Cornish engines, powered by Lancashire boilers, were used to permanently pump out the Great Spring and other sources of water from the tunnel. These were still in regular use until the 1960s, at which point they were replaced by electrically powered pumps. The Second Severn Crossing, which was built during the 1990s, crosses over the tunnel via a "ground level bridge" on the English side, near the Salmon Pool. This bridge is supported in such a way that no load is imposed on the tunnel. During that bridge's construction, the opportunity was taken to renew the concrete cap above the tunnel in the Salmon Pool. In 2002, two Class 121s were overhauled by LNWR, Crewe for use as a Network Rail emergency train that was stabled near Severn Tunnel Junction station. They were removed in 2008 having never been used. Car transport Before 1909 a few vehicles were transported through the tunnel on an informal basis. On 7 April 1909 the Great Western Railway started a formal service for the conveyance of motor cars through the tunnel. The vehicles were drained of petrol before being loaded, and refuelled at the destination. In the first year more than 300 cars were carried. The service was suspended for a time during the First World War but restarted in 1921. The charge in 1921 was £1 8s () on an open truck, or £2 8s () in a closed truck. The car shuttle train service would transport cars on rail trucks through the tunnel between Pilning and Severn Tunnel Junction. The service functioned as a rail-based alternative to the Aust Ferry, which was operated under an erratic timetable determined by the tides, or lengthy road journeys via Gloucester. The rail shuttle service was continued after the end of World War II, but was ultimately made redundant by the opening of the Severn Bridge in 1966, leading to its discontinuation shortly thereafter. ==Electrification==

As part of the 21st-century modernisation of the Great Western Main Line, the tunnel was prepared for electrification. While the structure provided good clearances and was therefore relatively easy to electrify, there was also a detracting factor in the form of the continuous seepage of water through the tunnel roof in some areas, which provided a key engineering challenge. The options of using either conventional tunnel electrification equipment or a covered solid beam technology were considered; supported by studies, it was decided to use the solid beam approach. Reportedly, the rigid rail is more robust, requires less maintenance, and is more compact than traditional overhead wires, and has been used in several other tunnels along the GWML. Following the completion of this work, which involved the installation of of copper contact wires using 1,700 vertical drop tubes and 857 anchoring points at a rough cost of £10 million to perform, the tunnel was reopened to regular traffic on 22 October 2016. However, less than two years later, another three-week closure of the tunnel was enacted after it was discovered that some of the recently installed overhead electrification equipment had already started to rust. To combat corrosion, aluminium wire was used, the first of its type in the United Kingdom. Electric trains began operating through the tunnel in June 2020. ==See also==