Railroad tie

While wooden ties dominate North American railways, concrete is widely used in other parts of the world.

A railroad tie (in US usage, generally known as a railway sleeper outside the US) 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%.^[1]

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.

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 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 something like timber sleepers had to be used. Two centuries later, stone sleepers would reappear in the form of slab track.

Wooden

A variant fastening of rails to wooden ties

Timber ties are usually of a variety of hardwoods, oak being a popular material.^[2] 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.^[2] 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.^[3]

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).

Concrete

Interest in concrete railroad ties was revived due to material shortages after World War II.

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.

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 could not be achieved until World War II, when the timbers used for ties were extremely scarce due to material shortages.^[4] 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 and Northeast Corridor by installing steel-reinforced concrete ties, updating some of the busiest rail lines in North America in order to facilitate higher operating speeds.^[5]

Steel

Steel Sleepers

In past times steel ties (sleepers) have suffered from poor design and increased traffic loads over their normal long service life. These aged and often obsolete designs limited load and speed capacity but can still, to this day, 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.

Modern day steel ties, particularly in North America where track loads are greater than in other continents, are not the steel ties of old. Newer steel ties handle heavy loads, have proven 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 100% recyclable and require up to 60% less ballast than concrete ties and up to 45% less than wood 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.

The steel ties’ cost benefits together with the ability to hold rail gauge, lower long-term maintenance costs, increase the life of other track components, reduce derailments and meet ever growing and stricter environment standards provide railroad companies with savings and capital to redirect to other areas of maintenance-of-way and business projects.

Plastic/Rubber Composite

In more recent times, a number of companies are selling composite railroad ties manufactured from recycled plastic resins,^[6] and recycled rubber. These ties are said to outlast the classic wooden tie, and are impervious to rot and insect attack,^[7][8][9] providing additional lateral stability^[7] 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.^[10]

Aside from the environmental benefits of using recycled material, plastic ties usually replace hardwood ties soaked in creosote, the latter being a toxic chemical,^[11] and are themselves recyclable.^[7] After several false starts that damaged the credibility of the composite tie industry due to 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.^[10] In 2007, the Long Island Rail Road began replacement of its wood ties to plastic on the Montauk Branch but has returned to using wood for spring 2010 track work.

Plastic/Rubber composite ties are used in other rail applications such as underground mining operations.^[12]

Urethane railroad ties are being used (as of 2008) in several German railway spurs such as the Leverkusen Chempark site of Bayer Chemical Company.^[13]

Some composite railroad tie manufacturers include Polywood ^[14], International Track Systems Inc.^[15], RTI - Recycle Technologies International, Inc.^[16], TieTek ^[17], Dynamic Composites ^[18], Axion International^[19], Integrico Composites^[20], Hansen Industries North America ^[21] and Performance Rail Tie, LP (PRT) ^[22].

Non conventional sleeper forms

Y shaped sleepers

Y sleeper track next to conventional track

An unusual form of sleeper is the Y shaped sleeper. First developed in 1983, Y steel sleepers have advantages and disadvantages compared to conventional steel sleepers. Compared to conventional sleepers the volume of ballast required is reduced due to the load spreading characteristics of the Y-sleeper.^[23] Noise levels are high but the resistance to track movement is very good.^[24] 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.

The cross section of the sleepers is an I-beam.^[25]

As of 2006 less than 1000km of Y-sleeper track had been built of which approximately ninety percent is in Germany.^[23]

Twin sleepers

The ZSX Twin sleeper is manufacturer by Leonhard Moll Betonwerke GmbH & Co KG and is a pair of two pre-stressed concrete sleepers longitudinally connected by four steel rods.^[26] The design is said to be suitable for regions 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.^[27]

Wide sleepers

Concrete monoblock sleepers have also been produced in a wider form (eg 57 cm (22 in)) such that there is no ballast between the sleepers; this wide sleeper increase lateral resistance and reduces ballast pressure.^[28][29][30] The system has been used in Germany^[31] where wide sleepers have also been used in conjunction with the GETRAC A3 ballastless track systems.^[32][33]

Bi-block sleepers

Bi-block (or twinblock) sleepers consist of two concrete rail supports joined by steel. Advantages include increase 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.^[34] This sleeper type is in common use in France^[35] Bi-block sleepers are also used in ballastless track systems.^[35]

Frame sleepers

Frame sleepers (German: Rahmenschwelle) comprise both lateral and longitudinal members in a single monolithic concrete casting.^[25] This system is in use in Austria;^[25] 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. ^[36]

Ladder track

In ladder track the "sleepers" are laid parallel to the rails and are several meters in length. 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.

Fastening rails to railroad ties

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.

Other uses

Wooden sleepers recycled as sculptures at Northfield station

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 Chemikalien-Verbotsverordnung (Chemicals Prohibition By-law), §1 and Annex, Parts 10 and 17.^[37]

Ballastless track

Slab track, System "Rheda 2000", prior to concrete pouring.

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.^[38]

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^[39] has the rail fastened directly to a concrete slab, about half a meter thick,^[40] 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".^[41]

Slab track, System "FF Bögl" on Nuremberg-Munich high-speed rail line

slab track at St Pancras station

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.^[40][42] 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.^[43]

Building a slab track is more expensive than building traditional ballasted track,^[42][43] which has slowed its introduction outside of high-speed rail lines. These layouts are not easy to modify after they are installed,^[43] and the curing time of the concrete makes it difficult to convert an existing, busy railway line to a ballastless setup.^[42]

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.^[40]

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,^[43] but can reduce noise and vibration by around 80%.^[44] 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.^[40]

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

References

External links

• Die Y-Stahlschwelle in der Schweiz Y sleepers in Switzerland - rack and normal railways , www.os.cd.cz