Why Leaves on the Line Actually Stop Trains

This black stripe is crushed leaves, thinner than a sheet of paper. Damp, it takes away a train's grip. Dry, it can hide the train from the signals. We follow 1S17, the 10:17 from King's Cross, braking into Durham on a grey October afternoon: how a passing train drags leaves onto its own rail, how the wheel crushes them into a film about 80 microns thick and what glues it there (lignin, cellulose and pectin), which trees are "the wrong type of leaves", why a little water is worse than a lot, what wheel slide protection and the sanders do (a controlled slide, not none), the Rule Book's word for a bad rail, and the second trick almost nobody knows: a dry film can insulate the wheels, so a track circuit loses the train. That's why the railway's treatment gel has steel in it. Then the price, the leaf-fall timetables, and the trees. 0:00 Intro 0:40 1S17, and the famous part 1:16 How a leaf becomes a film 3:03 Why damp is the danger 3:53 Braking on it 6:33 The dry trick 8:18 The paste, the price and the trees