work tasks can only be performed during temporary line closure, during which trains are normally diverted via . It is claimed 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) Company 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. As such, during the early 1870s, GWR's chief engineer, Sir John Hawkshaw, developed his design for this tunnel. On 27 June 1872, the company obtained an Act of Parliament which authorised the construction of the envisioned railway tunnel as a replacement for the ferry between Portskewett, Monmouthshire and Lew Passage, Gloucestershire. 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. The rate of early work on the tunnel was slow and gradual, but without major incident. By August 1877, only the shaft and a heading had been completed; accordingly, that same year, new contracts were issued for the digging of additional shafts at both sides of the Severn as well as new headings along the tunnel's intended route. As the civil engineer Thomas A. Walker, who was appointed as the contractor for the tunnel's construction, notes in his book, the GWR had expected the critical part of the work to be the tunnelling under the deep-water channel of the Shoots. However, the most substantial difficulties of the venture were encountered during October 1879, when, with only 130 yards (119 m) separating the main tunnel heading being driven from the Monmouthshire 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