The Sapperton Railway Tunnel is a railway tunnel near Sapperton, Gloucestershire in the United Kingdom. It carries the Golden Valley Line from Stroud to Swindon through the Cotswold escarpment. It was begun by the Cheltenham and Great Western Union railway in 1839 and taken over by the Great Western Railway in 1843, being completed in 1845. There are actually two tunnels, the main one at in length, and separated by a short gap, a second at . Construction, engineering, and maintenance difficulties The initial plans for the tunnel, dating from 1835, were unusual in that it was proposed to construct the tunnel on a curve, but this seems to have been abandoned before any construction was done; some works remain which are thought to relate to the approach route for the original line, but no excavations were made on that line for the tunnel itself. In 1836 "Mr Brunel" was appointed as engineer for the project; this refers to Isambard, but the involvement of "Mark (sic) Brunel" is also recorded. Brunel promised to get rid of the "objectionable" curve, and plans of the revised straight alignment were deposited in 1838. Preliminary shafts were dug, the work beginning in 1837, to ascertain the geological conditions, on the same straight alignment on which the tunnel was eventually built. In 1841 work began on four additional shafts of a larger diameter, quoted as 3 or 6 metres by different sources, plus a trial heading along the tunnel alignment. The line was opened in 1845. It was found that the intended route passed through a layer of fuller's earth, which was not sufficiently stable to allow construction of a tunnel. The plans were therefore revised to situate the tunnel in more stable strata at a shallower depth, at the expense of steeper gradients on the approaches. This also reduced the length of the tunnel from , on a 1 in 352 gradient in the 1836 plan, or on 1 in 330 in the 1838 plan (1 in 90 in the main tunnel, as built, and 1 in 57 on the approach to it), and so reduced construction costs; a 1950 article, based on the original contract and specification, quoted the directors as saying it would, "considerably diminish its length and expense" and asserted that this was the actual reason for the change to the shallower depth, rather than simply an effect of it. The