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Rail tracks are used on or Concrete sleepers. For illustrations see and [http://www.scalefour.org/resources/images/pandrol-w.jpg Rails, being made of steel, can carry heavier loads than any other material. Sleepers spread the load from the rails over the ground, and also serve to hold the rails a fixed distance apart (called the Gauge .) Rail tracks are normally laid on a bed of coarse stone chippings known as a '' Ballast '', which combines resilience, some amount of flexibility, and good Drainage ; however, track can also be laid on or into concrete (a slab track.) Across bridges, track is often laid on sleepers across longitudinal Timber s or longitudinal Steel Girder s. Additional detail on tracks used for Tram and Light Rail operations, as opposed to heavy rail, is available at Tramway Track . RAILWAY RAIL Unlike some other uses of Iron and Steel , railway rails are subject to very high stresses and have to be made of very high quality steel. It took many decades to improve the quality of the materials, including the change from iron to steel. Minor flaws in the steel that pose no problems with, say, reinforcing rods for buildings, can lead to broken rails and dangerous Derailment s when used on railway tracks. The rails represent a substantial fraction of the cost of a railway line. Only a small number of rail sizes are made by the steelworks at the one time, so a railway must choose the nearest suitable size. Worn, heavy rail from a mainline is often cascaded down to branchline, siding, or yard use. Rails are made in a large number of different sizes. Some common European rail sizes include:
Some common North American rail sizes include:
Rails in Canada , the United Kingdom , and United States are still described using imperial units. The examples in the diagram opposite are 113 and 95 pounds per yard (56 kg/m and 47 kg/m) respectively. However, in Australia they are now described in metric units and always have been on Mainland Europe . Early railroads sometimes used ''strap-iron rails'', which consisted of thin strips of iron strapped onto wooden rails. These rails were too fragile to carry heavy loads, but because the initial construction cost was less, this method was sometimes used to quickly build an inexpensive rail line. However, the long-term expense involved in frequent maintenance outweighed any savings. See also:
Axle Load By and large, the heavier the rails and the rest of the track, the heavier and faster the trains on those tracks can be. JOINTED TRACK There are different ways of joining rails together to form tracks. The traditional way of doing this was to bolt rails together in what is known as jointed track. In this form of track, lengths of rail, usually around 20 metres (60 feet) long, are laid and fixed to ''sleepers'' (U.K.) (''crossties'', or simply ''ties'' in North American parlance), and are joined to other lengths of rail with steel plates known as ''fishplates'' (U.K.) or ''joint bars'' (N.A.). Historically, North American railroads until the mid to late 20th Century used sections of rail that measured 39 feet (11.9 m) long so they could be carried to and from a worksite in conventional Gondola s, which often measured 40 feet (12.2 m) long; as car sizes increased, so did rail lengths. Fishplates or joint bars are usually 60 centimetres (2 feet) long, and are bolted through each side of the rail ends with bolts (usually four, but sometimes up to six.) Small gaps known as "expansion joints" are deliberately left between the rails to allow for expansion of the rails in hot weather. The holes through which the fishplate bolts pass are oval to allow for expansion. British practice was always to have the rail joints on both rails at the same place on each rail, while North American practice is to stagger them. Because of the small gaps left between the rails, when trains pass over jointed tracks, they make a "clickety clack, clickety clack" noise. Unless it is very well maintained, jointed track gives a fairly bumpy and uncomfortable ride, and is unsuitable for high speed trains because it is too weak. However, it is still used in many countries on lower speed lines, unimportant lines, and Siding s. Most railroad track in the United States is still of this type, however, and laid on timber ties; the lower speeds of American railroads make the disadvantages less apparent, and the abundant supply of timber in the United States makes its use for railroad ties much cheaper than in Europe. Jointed track is still extensively used in poorer countries, due to the cheaper construction costs and lack of modernisation of their railway systems. CONTINUOUS WELDED RAIL Most modern railways use continuous welded rail (CWR); in this form of track, the rails are Welded together by utilising the Thermite reaction to form one continuous rail that may be several kilometres long. Because there are few joints, this form of track is very strong, gives a smooth ride, and needs less maintenance. Because of its strength, trains travelling on welded track can travel at higher speeds and with less friction. Welded rails are more expensive to lay than jointed tracks, but are significantly cheaper to maintain. As mentioned earlier, rails expand in hot weather and shrink in cold weather. Because welded track has very few expansion joints, if no special measures are taken, it could become distorted in hot weather and cause a Derailment . To avoid this, welded rails are very often laid on Concrete sleepers, which are so heavy they hold the rails firmly in place, and with plenty of ballast to stop the sleepers moving. After new segments of rail are laid, or defective rails replaced (welded in), the rails are artificially heated to normal summertime temperatures so that they expand (this is called Stressing ). They are then fastened (clipped) to the sleepers in their expanded form. This process ensures that the rail will not expand much further in subsequent hot weather, and because they are firmly fastened, cannot shrink in cold weather either. However, if temperatures reach outside normal ranges (i.e. a hotter than usual summer), welded rails can become distorted. Joints are used in continuously welded rail when necessary; instead of a joint that passes straight across the rail, producing a loud noise and shock when the wheels pass over it, two sections of rail are cut at a steep angle and put together with a gap between them (a Breather Switch ). This gives a much smoother transition yet still provides some expansion room. METHODS OF FIXING RAIL TO SLEEPERS/TIES There are several methods used to fasten rail to wooden sleepers / ties. The worldwide standard type of rail used today is ''flat-bottomed rail'' ( Vignoles Rail ), which, as the name suggests, has a flat base and can stand upright without support. A flat-bottomed rail has a cross-section like that of an upside-down 'T' and is usually held to the sleeper with a ''baseplate'', a metal plate attached to the sleeper; although for cheap construction FB rails can be laid directly onto the sleepers. Modern sleepers can be made of reinforced concrete and pressed steel, with rubber pads inserted between the sleeper and rail. This is done for two reasons: to give a smoother ride and to prevent the sleeper from shorting the Track Circuit , a low voltage passed through the rails for signalling purposes. This is different from a "traction current," which powers electric trains. See also {Link without Title} A variety of different types of heavy-duty clips are used to fasten the rails to the underlying baseplate, one common one being the ''Pandrol'' fastener (Pandrol clip), named after its maker, which is shaped like a sturdy, stubby paperclip. [http://www.pandroluk.com/casestudies/prj31_bdy.htm and [http://www.scalefour.org/resources/images/Pandrol1.jpg] North American practice normally uses '' Rail Spike s'', which are fundamentally very large nails with bent-over heads to clasp the flat-bottomed rail. These are cheaper and simpler to install but can loosen if the tie rots, much more easily than the British chair (a type of baseplate) does. This is mitigated by using very large and solid Creosoted ties or using another rot-proofing Preservative . See also Timber Treatment . In traditional British practice, cast metal ''chairs'' were Screw ed to the sleepers, which took a style of rail known as ''bullhead'' that was somewhat figure-8 in cross-section — wider at top and bottom (known as the ''head'' and ''foot'' respectively) and smaller in the middle (the ''web''). ''Keys'' (wedges of wood or sprung steel) were then driven in between chair and rail to hold it in place. This was common practice on British railways until the 1950s, but is now largely obsolete. The idea behind ''bullhead'' rails was that because both the top and bottom of the rails were the same shape, when one side of the rail became worn, the rail could be turned over to the unused side, thus extending the rail's lifespan. In practice, ''bullhead'' rails have a flat base (narrower than flat-bottomed rail), and the top part has curved edges that fit the profile of the train wheels. In recent years, methods have been developed to put tracks on concrete without using conventional sleepers or track ballast. While this method's construction cost is high, this system is expected to have significantly lower maintenance cost than conventional tracks. It is mainly used on high-speed lines and in tunnels, where maintenance access is difficult. |
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