The Clickety-Clack and the Heat

Early rails were short and brittle. In the 1790s, cast iron versions, about four feet long, were being bolted down. By 1820, wrought iron had stretched them to fifteen feet. The first steel rails arrived in 1857, and over the following decades, the standard length crept up from thirty feet to sixty. Sixty feet, eighteen metres, became the length some of us grew up hearing.

Hearing is the right word. Those rails were laid end to end, with a small gap between each, held together by fishplates. The gap was deliberate. Steel grows when it warms, and a rail with nowhere to go will fight back, so the engineers left it room. Every time a wheel crossed one of those gaps, it gave a knock, and with two rails and a regular wheelbase, the knocks fell into a rhythm. Clickety-clack. It was the sound of a problem being managed, though it became the sound of the railway itself, the thing people remember from sleeper trains and childhood holidays.

The problem is simple physics. Steel expands by roughly 12 parts per million for every degree Celsius. That sounds trivial. Run the numbers along a real railway, and it stops being trivial. Take a five-kilometre length of rail. Warm it by a single degree, and it grows by about six centimetres. Warm it by ten degrees, an ordinary swing on an English afternoon, and it grows by sixty centimetres. Two feet. A rail with nowhere to expand does not lose that energy. It pushes to the side instead, and the track snakes out of line. Railwaymen call it a sun kink, or a buckle. On the worst days, a train has to be stopped before it reaches one. Imagine in today’s heat of 37C!

The gaps solved the buckling, but they also wore everything out. The joint was the weak point, hammered by every passing wheel, and it brought the rough ride and the maintenance bill along with it.

The answer was to get rid of the joints, which sounds like asking for the buckling back. But the trick is in the laying. The first long welded rail in Britain, almost a mile of it, went down on the East Coast Main Line just south of Carlton-on-Trent in 1957. From 1960, the practice spread, and by about 1968, continuous welded rail was the standard being rolled out across the network. Instead of leaving room to expand, the engineers clip the rail hard to heavy concrete sleepers and pre-stretch it as they lay it, setting it stress-free at around 27 degrees. On a hot day, the steel cannot move, so the warmth becomes pressure held in place rather than movement let loose. On a cold day, the same trick stops it from pulling apart.

So the modern track is smoother, quieter, and lasts far longer. The clickety-clack has mostly gone, traded for a hush. Honestly, that’s a bit of a shame!