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TRAIN WHEEL FLANGE RAIL HEAD WEB FOOT Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format. New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address. THE LAST WORD On the line Why do rails for trains generally have such a similar cross section (see diagram)? n The train rail has evolved a long way over 250 years, but hundreds of different rail sections are still used throughout the world. To the untrained eye they might all look the same, much in the way that cars all look the same – having four wheels, a windscreen and headlights. But on closer inspection there are differences between the rail used, for example, on the London Underground and on mainline UK track. And heavy-freight track is subtly different yet again. Rails were an “L” section to hold the trains in place 250 years ago, but with the introduction of wheel flanges to keep trains on the track, rail design got interesting. Engineers found that it was best to support rails on sleepers (called “ties” in North America) to stop them sinking into the ground. The primary function of a rail became to carry the heavy load in the gaps between the sleepers. The basic shape of a rail section is explained by three functional traits. Cost dictates that the “web” – the vertical element – be as thin as possible, much like in the structural I-beams for buildings. The rail “head” has to be chunky because this is the bit that wears out under the rolling wheels, and the “foot” must be wide to give stability. Rail evolution has included some great ideas, such as the symmetrical bullhead rail, which can be turned upside down as it wears out, but this proved impractical. Even today, new rails are being designed to reduce noise or improve reliability, but they will always look much the same, with a head, a web and a foot. Hugh Hunt Cambridge, UK n Some early mine railways used external wheel flanges, but these came to naught, as did systems using flanged, L-shaped rails and flat wheels. They made it hard to push or pull the wagon wheels around curves, where the wheels rubbed against the rails. The invention of the coned wheel has not been accurately traced, but train pioneer George Stephenson already understood the principle. He knew that “edge rails”, the kind we recognise today, combined with slightly conical train wheels – with the larger diameter being on the inside – would allow trains to self-centre on the track, minimising contact with the rails. A prerequisite for this is that the wheels are fixed rigidly on the axle, which rotates with them, so that as one wheel runs on the smaller diameter of its cone, the opposite wheel is forced onto its larger diameter and steers the whole axle back towards a centred position. The resulting motion reduces contact between the wheel flange and the rail cheek (the side of the head), even in curves. The rail can also be mounted with a slight inward inclination, as in Europe, further enhancing self-steering. High-speed rail tracks show that there is only a very narrow shiny contact strip on the rail top, while the rest remains rusty. Even so, there is some flange contact, especially in switches and sharp curves, and this tends to grind away at the rail and the wheel. Were both to be given a sharp angular corner, it would quickly wear, leaving the rail with a rounded “gauge corner” and the wheel with a concave profile at the junction between rolling surface and flange. Engineers found that the loss of metal is minimised if both components are made with the profile they will adopt when partially worn. The rolling surfaces of wheel and rail do suffer wear and have to be corrected from time to time – the wheel by grinding or by use of a large lathe, and the rail by grinding or replacement. Fatigue cracks appearing in the rail head may be worn away by passing traffic, but hardened rails used in some curves may need these cracks ground off before they start to propagate. Such cracks, left uncorrected, have led to at least one serious accident – a crash at Hatfield, UK, in 2000 that killed four people and injured dozens more. The symmetrical rail profile simply avoids the rails being left- or right-handed, which would complicate their installation. Nick Cory Rail Systems Engineer Bombardier Transportation Germany This week’s question FEEL THE HEAT When I open my dishwasher, which has a dry heat/dry finish, the china, glassware and cutlery are dry, but plastic containers are still covered in drops of water. This is regardless of position or proximity in the wash. Why is the plastic wet, when the rest is dry? David Morrissey Sydney, Australia “High-speed rail tracks show that there is only a very narrow shiny contact strip on the rail top” “With the introduction of wheel flanges to keep trains on the track, rail design got interesting” Last words past and present, plus questions, at last-word.com A new collection: the usual insight, ingenuity and wit – this time with full colour photographs Available from booksellers and at newscientist.com/orangutans Why are orangutans orange?
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