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A railway sleeper or railroad tie is a rectangular object used as a base for railroad tracks. Ties are members generally laid transverse to the rails, on which the rails are supported and fixed, to transfer the loads from rails to the ballast and subgrade, and to hold the rails to the correct gauge. Traditionally, ties have been made of wood, but concrete is now widely used. Steel ties and plastic composite ties are currently used as well, although far less than wood or concrete ties. As of January 2008, the approximate market share, in North America, for traditional and wood ties was 91.5%, whereas the approximate combined market share for (all) concrete, steel, azobe (exotic hardwood) and plastic composite ties was 8.5%. Timber ties are usually of a variety of hardwoods, oak being a popular material. Some lines use softwoods, sometimes due to material necessity; while they have the advantage of accepting treatment more readily, they are more susceptible to wear. They are often heavily creosoted. Creosote treating can reduce insect infestation and rot. However, creosote is also carcinogenic and environmentally damaging. Less often, ties are treated with other preservatives, although some timbers (such as sal) are durable enough that they can be used untreated. Problems with wood ties include rot, splitting, insect infestation, plate-cutting (abrasive damage to the tie caused by lateral motion of the tie plate) and spike-pull (where the spike is gradually worked out and loosened from the tie). Ties are normally laid on top of track ballast, which supports and holds them in place, and provides drainage and flexibility. Heavy crushed stone is the normal material for the ballast, but on lines with lower speeds and weight, sand, gravel, and even ash from the fires of coal-fired steam locomotives have been used. Approximately 3000 ties are used per mile of railroad track. Ties/sleepers are set much closer together in the USA, where rails are traditionally joined to the track by a railroad spike rather than the substantial iron/steel chairs used in Europe. Concrete ties have become more common mainly due to greater economy and better support of the rails under high speed and heavy traffic than wooden ties. In early railway history, wood was the only material used for making ties in Europe. Even in those days, occasional shortages and increasing cost of wood posed problems. This induced engineers to seek alternatives to wooden ties. As concrete technology developed in the 19th century, concrete established its place as a versatile building material and could be adapted to meet the requirements of railway industry. [READ THE REST OF THIS ARTICLE]
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