Climbing almost non-stop at the ruling grade of 1 in 66 or 1.5% the line has almost continuous curves of radius. Near Cougal, the alignment finally runs out of valley, and has nowhere to go. Fortunately a convenient hill allows the line to circle back on itself so that it climbs without having to make any forward progress. Soon after the Border Tunnel under the watershed is reached the line descends on the other side. The spiral has two short tunnels, one where it passes under itself, plus another through a small spur in the hillside. The tunnel at the border summit is approximately long, broken up into three separate tunnels. A lower summit, perhaps without the spiral, would be possible with a considerably longer and more expensive summit tunnel, with the possibility of problems with fumes for both the original steam locomotives and the replacement diesel engines. The Cougal Spiral is easily viewed from parts of the Lions Road. The structure has been heritage listed due to its historical, scientific and architectural rarity. It opened on 29 August 1930 as part of the extension of the North Coast line from Kyogle to South Brisbane. The Cougal Railway Spiral and Landscape consists of the following elements:
an underbridge, constructed of five steel spans RSJ on concrete piers, located at from Central;
a concrete tunnel, located from from Central;
a concrete tunnel, located from from Central;
the Border Loop crossing, located at from Central ; and
a tunnel on the border with Queensland, located from from Central.
The spiral is formed by the whole section of line from Cougal to the Border loop.
History
The Border Loop opened on 29 August 1930, connecting NSW and Queensland by rail via Richmond Gap.
Heritage listing
As at 19 July 2013, the Cougal to Border Loop section of the North Coast Line was a notable engineering achievement, revealing a twentieth century engineering solution that allows trains to pass through the steep topography at the NSW/Queensland border. Border Loop railway formation and landscape was listed on the New South Wales State Heritage Register on 2 April 1999 having satisfied the following criteria. The place is important in demonstrating the course, or pattern, of cultural or natural history in New South Wales. The completion of the Cougal to Border Loop section of the North Coast Railway was a major engineering feat in the first half of the twentieth century, being one of several major engineering projects in NSW that allowed continuous railway access between Sydney and Brisbane. The place is important in demonstrating aesthetic characteristics and/or a high degree of creative or technical achievement in New South Wales. The combination of tunnels, crossing loops, the underbridge and spiral together form a technically significant example of an innovative and successful railway engineering solution. The place possesses uncommon, rare or endangered aspects of the cultural or natural history of New South Wales. The railway spiral at Border Loop is one of only two railway spirals in NSW.
