The Cougal Spiral is a feature of the North Coast Railway in Australia that connects New South Wales with Queensland through Richmond Gap. The railway lines needs to climb at a steady ruling gradient from Kyogle to the summit at a tunnel at the border between the two states. The border is also at the watershed. The line is single tracked.
Climbing almost non-stop at the ruling grade of 1 in 66 or 1.5% (compensated) the line has almost continuous curves of 240 m (790 ft) 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 30 meters (98 ft) without having to make any forward progress. Soon after the Border Tunnel under the watershed is reached, and 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 summit is 1,600 meters (5,200 ft) long. 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.[1]