1.one of the cross braces that support the rails on a railway track"the British call a railroad tie a sleeper"
voir la définition de Wikipedia
barre : pièce longue et rigide (fr)[ClasseParExt.]
voie de chemin de fer (fr)[Thème]
railway sleeper; tie; railroad tie; crosstie[ClasseHyper.]
voie ferrée (fr)[DomainDescrip.]
railroad tie (n.)
A railroad tie/railway tie (North America), or railway sleeper (Europe) is a rectangular support for the rails in railroad tracks. Generally laid perpendicular to the rails, ties transfer loads to the track ballast and subgrade, hold the rails upright, and keep them spaced to the correct gauge.
Railroad ties were traditionally made of wood, but pre-stressed concrete is now widely used especially in Europe and Asia. Steel ties are common on secondary lines in the UK; plastic composite ties are also employed, although far less than wood or concrete. As of January 2008, the approximate market share in North America for traditional and wood ties was 91.5%, the remainder being concrete, steel, azobé (red ironwood) and plastic composite.
Coarse aggregate is the standard material for track ballast, which provides drainage and resilience. On lines with lower speeds and axle-weights, sand, gravel, and even coal ash from the fires of steam locomotives have been used.
Up to 3000 ties are used per mile of railroad track in the USA, 2640 per mile (30 per 60ft rail) on main lines in the UK. Rails in the USA may be fastened to the tie by a railroad spike; iron/steel baseplates screwed to the sleeper and secured to the rail by a proprietary fastening system such as a Vossloh or Pandrol are commonly used in Europe.
The type of sleeper used on the predecessors of the first true railway (Liverpool and Manchester Railway) consisted of a pair of stone blocks laid into the ground, with the chairs holding the rails fixed to those blocks. One advantage of this method of construction was that it allowed horses to tread the middle path without the risk of tripping. In railway use with ever heavier locomotives, it was found that it was hard to maintain the correct gauge. The stone blocks were in any case unsuitable on soft ground, where timber sleepers had to be used.
A variety of softwood and hardwoods timbers are used as ties, oak, jarrah and karri being popular hardwoods, although increasingly difficult to obtain, especially from sustainable sources. Some lines use softwoods, including Douglas fir; while they have the advantage of accepting treatment more readily, they are more susceptible to wear but are cheaper, lighter (and therefore easier to handle) and more readily available.  Softwood is treated, historically using creosote, but nowadays with other less-toxic preservatives to improve resistance to insect infestation and rot. New boron-based wood preserving technology is being employed by major US railroads in a dual treatment process in order to extend the life of wood ties in wet areas. Some timbers (such as sal, mora, jarrah or azobé) are durable enough that they can be used untreated.
Problems with wood ties include rot, splitting, insect infestation, plate-cutting (known as chair shuffle in the UK)(abrasive damage to the tie caused by lateral motion of the tie plate) and spike-pull (where the spike is gradually loosened from the tie). For more information on wood ties the Railway Tie Association maintains a comprehensive website devoted to wood tie research and statistics.
Interest in concrete railroad ties increased after World War II following advances in the design, quality and production of pre-stressed concrete. Concrete ties were cheaper and easier to obtain than timber and better able to carry higher axle-weights and sustain higher speeds. Their greater weight ensures improved retention of track geometry especially when installed with continuous-welded rail. Concrete sleepers have a longer service life and require less maintenance than timber due to their greater weight which helps them remain in the correct position longer. Concrete sleepers need to be installed on a well-prepared subgrade with an adequate depth on free-draining ballast to perform well.
In 1877, M. Monnier, a French gardener, suggested that concrete could be used for making ties for railway track. Monnier designed a tie and obtained a patent for it, but it was not successful. Designs were further developed and the railways of Austria and Italy used the first concrete ties around the turn of the 20th century. This was closely followed by other European railways.
Major progress was not achieved until World War II, when the timbers used for ties were scarce due competition from other uses, such as mines. Following research carried out on French and other European railways, the modern pre-stressed concrete tie was developed. Heavier rail sections and long welded rails were also being installed, requiring higher-quality ties. These conditions spurred the development of concrete ties in France, Germany and Britain, where the technology was perfected. On the highest categories of line in the UK (those with the highest speeds and tonnages) pre-stressed concrete sleepers are the only ones permitted by Network Rail standards.
Most European railways also now use concrete bearers in switches and crossing layouts due to the longer life and lower cost of concrete bearers compared to timber, which is increasingly difficult and expensive to source in sufficient quantities and quality.
On November 8, 2011, the US Federal Railroad Administration (FRA) put into effect new regulations on concrete ties, with notices published by the FRA in the April 1 and September 9, 2011 U. S. Federal Register. The FRA notices say that the need for the new rules was shown by the derailment of an Amtrak train near Home Valley, Washington on April 3, 2005, which according to the U.S. National Transportation Safety Board was caused in part by excessive concrete tie abrasion. To be counted as a good tie under FRA regulation 213.109(d)(4), a concrete ties shall not be deteriorated or abraded under the rail to a depth of one-half inch or more. Limits on other types of concrete tie deterioration are also given.
Steel sleepers are formed from pressed steel and are trough-shaped in section. The ends of the sleeper are shaped to form a "spade" which increases the lateral resistance of the sleeper. Housings to accommodate the fastening system are welded to the upper surface of the sleeper. Steel sleepers are now in widespread use on secondary or lower-speed lines in the UK where they have been found to be economical to install due their ability to be installed on the existing ballast bed. Steel sleepers are lighter in weight than concrete and able to stack in compact bundles unlike timber. Steel sleepers can be installed onto the existing ballast, unlike concrete sleepers which require a full depth of new ballast. Steel ties are 100% recyclable and require up to 60% less ballast than concrete ties and up to 45% less than wood ties.
Historically, steel ties (sleepers) have suffered from poor design and increased traffic loads over their normally long service life. These aged and often obsolete designs limited load and speed capacity but can still be found in many locations globally and performing adequately despite decades of service. There are great numbers of steel ties with over 50 years of service and in some cases they can and have been rehabilitated and continue to perform well. Steel ties were also used in specialty situations, such as the Hejaz Railway in the Arabian Peninsula, which had an ongoing problem with Bedouins who would steal wooden ties for campfires.
Modern steel ties handle heavy loads, have a proven record of performance in signalized track, and handle adverse track conditions. Of high importance to railroad companies is the fact that steel ties are more economical to install in new construction than creosote-treated wood ties and concrete ties. Steel ties are utilized in nearly all sectors of the worldwide railroad systems including heavy-haul, class 1’s, regional, shortlines, mining, electrified passenger lines (OHLE) and all manner of industries.
Notably, steel ties (bearers) have proven themselves over the last few decades to be advantageous in turnouts (switches) and provide the solution to the ever-growing problem of long timber ties for such use.
In more recent times, a number of companies are selling composite railroad ties manufactured from recycled plastic resins, and recycled rubber. Manufacturers claim a service life comparable with wooden ties, and that the ties are impervious to rot and insect attack, and can be modified to provide additional lateral stability while otherwise exhibiting properties similar to their wooden counterparts in terms of damping impact loads and sound absorption.
Aside from the environmental benefits of using recycled material, plastic ties usually replace timber ties soaked in creosote, the latter being a toxic chemical, and are themselves recyclable. Plastic/Rubber composite ties are used in other rail applications such as underground mining operations.
It was announced on the Network Rail website on 19 May 2011 that a trial of recycled plastic (Zyplex) sleepers is to take place in the UK from Summer 2011.
An unusual form of sleeper is the Y shaped sleeper. First developed in 1983. Compared to conventional sleepers the volume of ballast required is reduced due to the load spreading characteristics of the Y-sleeper. Noise levels are high but the resistance to track movement is very good. For curves the three point contact of a Y steel sleeper means that an exact geometric fit cannot be observed with a fixed attachment point.
As of 2006 less than 1000 km of Y-sleeper track had been built of which approximately ninety percent is in Germany.
The ZSX Twin sleeper is manufacturered by Leonhard Moll Betonwerke GmbH & Co KG and is a pair of two pre-stressed concrete sleepers longitudinally connected by four steel rods. The design is said to be suitable for track with sharp curves, track subject to temperature stress such as that operated by trains with eddy brakes, bridges and as transition track between traditional track and slab track or bridges.
Concrete monoblock sleepers have also been produced in a wider form (e.g. 57 cm (22 in)) such that there is no ballast between the sleepers; this wide sleeper increases lateral resistance and reduces ballast pressure. The system has been used in Germany where wide sleepers have also been used in conjunction with the GETRAC A3 ballastless track systems.
Bi-block (or twinblock) sleepers consist of two concrete rail supports joined by a steel bar. Advantages include increased lateral resistance and lower weight than monobloc concrete sleepers, as well as elimination of damage from torsional forces on the sleeper centre due the more flexible steel connections. This sleeper type is in common use in France, and are used on the high-speed TGV lines. Bi-block sleepers are also used in ballastless track systems.
Frame sleepers (German: Rahmenschwelle) comprise both lateral and longitudinal members in a single monolithic concrete casting. This system is in use in Austria; in the Austrian system the track is fastened at the four corners of the frame, and is also supported midway along the frame. Adjacent frame sleepers are butted close to each other. Advantages of this system over conventional cross tie sleepers are reduced ballast pressure (up to half), increased lateral resistance, and increased support of track. In addition, construction methods used for this type of track are similar to those used for conventional track.
In ladder track the "sleepers" are laid parallel to the rails and are several meters long. The structure is similar to Brunel's baulk track; these longitudinal sleepers can be used with ballast, or with elastomer supports on a solid non-ballasted support.
Various methods exist for fixing the rail to the sleeper (railroad tie). Historically spikes gave way to cast iron chairs fixed to the sleeper, more recently springs (such as Pandrol clips) are used to fix the rail to the sleeper chair.
In recent years, wooden railroad ties have also become popular for gardening and landscaping, both in creating retaining walls and raised-bed gardens, and sometimes for building steps as well. Traditionally, the ties sold for this purpose are decommissioned ties taken from rail lines when replaced with new ties, and their lifespan is often limited due to rot. Some entrepreneurs sell new ties. Due to the presence of wood preservatives such as coal tar, creosote or salts of heavy metals, railroad ties introduce an extra element of soil pollution into gardens and are avoided by many property owners. In the UK, new oak beams of the same size as standard railroad ties, but not treated with dangerous chemicals, are now available specifically for garden construction. They are about twice the price of the recycled product. In some places, railroad ties have been used in the construction of homes, particularly among those with lower incomes, especially near railroad tracks, including railroad employees. They are also used as cribbing for docks and boathouses.
In Germany, use of wooden railroad ties as building material (namely in gardens, houses and in all places where regular contact to human skin would be likely, in all areas frequented by children and in all areas associated with the production or handling of food in any way) has been prohibited by law since 1991 because they pose a significant risk to health and environment. From 1991 to 2002, this was regulated by the Teerölverordnung (Carbolineum By-law), and since 2002 has been regulated by the Chemikalien-Verbotsverordnung (Chemicals Prohibition By-law), §1 and Annex, Parts 10 and 17.
First such tracks were mountain railways (like Pilatus railway, built in 1889) with rails attached directly to the mountain rock. From the late 1960s onwards, German, British, Swiss and Japanese railroads experimented with alternatives to the traditional railway tie in search of solutions with higher accuracy and longevity, and lowered maintenance costs.
This gave rise to the ballastless railway track, especially in tunnels, high-speed rail lines and on lines with high train frequency, which have high stress imposed on trackage. Paved concrete track has the rail fastened directly to a concrete slab, about half a meter thick, without ties. A similar but less expensive alternative is to accurately position concrete ties and then pour a concrete slab between and around them; this method is called "cast-in precast sleeper track".
These systems offer the advantage of superior stability and almost complete absence of deformation. Ballastless track systems incur significantly lower maintenance costs compared to ballasted track. Due to the absence of any ballast, damage by flying ballast is eliminated, something that occurs at speeds in excess of 250 km/h (150 mph). It is also useful for existing railroad tunnels; as slab track is of shallower construction than ballasted track, it may provide the extra overhead clearances necessary for converting a line to overhead electrification, or for the passage of larger trains.
Building a slab track is more expensive than building traditional ballasted track, which has slowed its introduction outside of high-speed rail lines. These layouts are not easy to modify after they are installed, and the curing time of the concrete makes it difficult to convert an existing, busy railway line to a ballastless setup.
Slab track can also be significantly louder and cause more vibration than traditional ballasted track. While this is in some part attributable to slab track's decreased sound absorption qualities, a more significant factor is that slab track typically uses softer rail fasteners to provide vertical compliance similar to ballasted track; these can lead to more noise, as they permit the rail to vibrate over a greater length.
Where it is critical to reduce noise and vibration, the concrete slab can be supported upon soft resilient bearings. This configuration, called "floating slab track", is expensive and requires more depth or height, but can reduce noise and vibration by around 80%.[clarification needed] Alternatively, the rail can be supported along its length by an elastic material; when combined with a smaller rail section, this can provide a significant noise reduction over traditional ballasted track.
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