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Railroad tie
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A railroad tie, cross tie, or railway sleeper is a rectangular object used as a base for railroad tracks. Sleepers 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.
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Encyclopedia
A railroad tie, cross tie, or railway sleeper is a rectangular object used as a base for railroad tracks. Sleepers 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, however far less than wood or concrete ties. As of January 2008, the approximate market share, in North American, 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%.
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.
Types
Stone Block
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 to allow horses to tread a 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 something like timber sleepers had to be used. Two centuries later, stone sleepers would reappear in the form of slab track.
Wooden
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 or, less often, treated with other preservatives, although some timbers (such as sal) are durable enough that they can be used untreated.
The main problem with wood is its tendency to rot, particularly near the points where the ties are fastened to the rails. The timber industry has responded to decreased use of timber by promoting its advantages; wooden ties still dominate the North American market.
Concrete
Concrete ties have become more common mainly due to greater economy and better support of the rails under heavy traffic. 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.
In 1877, Mr. Monnier, a French gardener and inventor of reinforced concrete, 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 could not be achieved until World War II, when the timbers used for ties were extremely scarce due to material shortages. Due to research carried out on French and other European railways, the modern concrete tie was developed. Heavier rail sections and long welded rails were also being produced, requiring higher-quality ties. These conditions spurred the development of concrete ties in France, Germany and Britain, where the technology was perfected.
Toward the end of the 1990s, the Long Island Rail Road, followed by Amtrak, began rehabilitation of their lines in the New York metropolitan area by installing steel-reinforced concrete ties, updating some of the busiest rail lines in North America.
Steel
Steel ties, which are relatively light in weight, are sometimes used for sidings and temporary tracks. They have the advantages of being relatively free from decay and attack from insects and providing excellent gauge restraint, but are prone to rust and wear at the rail seat; they also require frequent replacement and tightening of fastenings. They are generally unsuited to railway lines carrying vehicles traveling at over 60 mph (100 km/h) because they provide no damping, all force being transmitted to the underlying track ballast. Prefabricated, all-metal "Jubilee" track, which was developed in the late nineteenth century for use in quarries and the like and saw some use in major civil engineering projects, is one example of steel ties in use.
Plastic/Rubber Composite
In more recent times, a number of companies are selling composite railroad ties manufactured from recycled plastic resins, and recycled rubber. These ties are said to outlast the classic wooden tie, and are impervious to rot and insect attack, provide additional lateral stability, while otherwise exhibiting properties similar to their wooden counterparts in terms of damping impact loads and sound absorption. More pragmatically, they offer the advantage of being able to replace wooden ties piecemeal; concrete ties use different equipment and require that the trackbed be all concrete or none.
Aside from the environmental benefits of using recycled material, plastic ties usually replace hardwood ties soaked in creosote, the latter being a toxic chemical, and are themselves recyclable. After several false starts that damaged the credibility of the composite tie industry—manufacturers that found themselves unable to deliver more than sample quantities—plastic ties have gained some acceptance from railroads, the Union Pacific's million-tie order being seen as something of a breakthrough for the industry. Starting in 2007, the Long Island Rail Road began replacement of its wood ties to plastic on the Montauk Branch.
Plastic/Rubber composite ties are used in other rail applications such as underground mining operations.
Urethane railroad ties are being used (as of 2008) in several German railway spurs such as the Leverkusen Chempark site of Bayer Chemical Company.
Some notable composite railroad tie manufacturers:
- Polywood
- International Track Systems Inc.
- RTI - Recycle Technologies International, Inc.
- TieTek
- Dynamic Composites
- Axion International
- Integrico Composites
Tubular Modular Track
In Tubular Modular Track (TMT), the "sleepers" are laid parallel to the rails and are continuous, rather like Brunel's baulk track. Every so often the two concrete sleepers are connected..
Fastening rails to ties
Ties are laid across the ballast at intervals of about two feet (roughly 3,000 per mile). The rails are then laid atop the ties, perpendicular to them. If the ties are wood, tie plates are then set atop the ties on the rail flange, then spikes or bolts are driven through the tie plates into the ties to clamp down the rails. Historically, railroads with American-inspired technology have used driven rail spikes to hold the rail to the tie, while European railways favor square-headed bolts that are screwed into the wood. For concrete ties, steel clips (for example the Pandrol clip) are often used to fasten the rails. After this is done, additional ballast is then added to fill the spaces between and around the ties to anchor them in place.
The ties then act as anchors and spacers for the rails, while providing a slight amount of give to accommodate weather and settling. The ties are "floating" in the top of the ballast. Failure of a single tie is generally insignificant to the usability and safety of the rails. Some railroads used a "date nail" coded to identify the age of the railroad tie (that was usually laid down in sections) by hammering it into the railroad tie after installation for maintenance purposes; this practice is almost unheard of today.
Rails lie somewhat freely in tie plates and sliding movement of the rail through the plate is possible, leading to creeping rails or misaligned or unevenly spaced ties. To prevent this, anchors may be placed transversely under the rail at each side of the tie to prevent slippage of the rail and the tie relative to each other. The tie anchor is usually a spring-loaded clip placed with a hammer-blow (driven) or with a special lever (wrench).
Other uses
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. However, 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 those residing near railroad tracks, including railroad employees. They are also used as cribbing for docks and boathouses.
The Spanish artist Agustín Ibarrola has used recycled ties from RENFE in several projects.
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 (Chemicals Prohibition By-law), §1 and Annex, Parts 10 and 17.
Ballastless track
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%. 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.
See also
- Rail tracks
- Track ballast
- Contaminated wooden sleepers may be disposed of in portland cement kilns.
- "The railway sleeper: 50 years of pretensioned prestressed concrete", H.P.J.Taylor The Structural Engineer August 1993 pp281-288
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