Climbing almost non-stop at the ruling grade of 1 in 66 or 1.5% (compensated) 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.