Contact Patch

Transport technology from the ground up



Sitting in a station café, you can bask in the romance of railway travel and watch the trains come and go. In the distance beyond lies an alien world of tracks that criss-cross one another through a gash in the city landscape. Railway engineers often refer to the railway track as a ‘road’. But it is very different from the highway that passes in front of your house. The train wheels roll on two steel bands each with a surface only a few millimetres wide. A large part of the railway operator’s budget is devoted to their upkeep: looking for flaws, grinding the surface, and adjusting their alignment in order to smooth the ride. The ride is sometimes spectacular. Being relatively narrow, railway tracks can penetrate the landscape, rising on viaducts and tunnelling through hills in a way that would be too costly for a motorway. On a train ride you can take in scenes that a car driver cannot: for example the eastbound journey from London to Thanet where the line sweeps across the North Downs, descends and curves towards the Medway Estuary then rumbles across the bridge towards Rochester Castle on the far shore.

Railway travel is not always a magical experience, though. Things can go wrong, and on rare occasions, disastrously so. On 9 June 1865, the Folkestone-to-London boat train derailed at about 50 miles per hour (80km/h) while crossing the Beult River viaduct, a modest structure about 3 metres high. Seconds earlier, after passing through Headcorn railway station the driver had seen a man waving a red flag. But he could not stop the train before it rolled onto the viaduct, where a length of track had been removed during engineering works. Seven of the coaches fell into the river bed, killing ten passengers and injuring forty. Among the passengers was Charles Dickens, who was travelling with a friend. Dickens survived and helped to look after the victims, some of whom died before they could be taken to hospital. Deeply distressed by the accident, he could not speak for days. According to his son, he never fully recovered before his death five years later.

Since then, there have been rail crashes in many countries throughout the world, some far more serious in terms of human casualties. But they all remind one of something that is peculiar to railway travel: the system relies on the condition of the track. Even with the rails firmly in position, there is a small margin between running normally and careering out of control: about 8 millimetres, in a sense that will become clear as we go along. The interaction between the wheel and rail is more interesting and more complex than you might imagine, and it has the potential to cause problems. The behaviour of conventional railway wheels, benign at low speeds when they steer themselves round curves in the track, becomes erratic when the speed of a train rises above a critical level. So in what follows, we’ll start by looking at what happens within the contact patch between wheel and rail and then work our way upwards.