Railroad tie
From Wikipedia, the free encyclopedia
A railroad tie, cross tie, or 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 sub grade below, and to hold the rails to the correct gauge.
Traditionally, ties have been made of wood, but concrete is now widely used, steel has been used, and plastic has been tried.
Ties are normally laid on top of track ballast, used to support and hold the sleepers in place, provide drainage, and have some flexibility. Heavy crushed stone is the normal material for this, but on lines with lower speeds and weight, sand, gravel, and even ash from the fires of coal-fired steam locomotives have been used.
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[edit] Wooden sleepers
Timber sleepers are usually of a variety of hardwoods, and are often heavily creosoted or, less often, treated with other preservatives.
The problems with wood are the tendency to rot, particularly around the fastenings used to hold the rails to them.
The timber industry has responded to the decreased use of timber by promoting its advantages.[1]
[edit] Concrete sleepers
Concrete sleepers have become more common mainly due their greater economy and better support of the track under heavy traffic. In the early period of history of railways, wood was the only material used for making sleepers in Europe. Even in those days, occasional shortages of wood and the increasing price of wood posed problems. This induced engineers to find an alternative to wooden sleepers. With the development of concrete technology in the 19th century, concrete had established its place as a versatile building material and could be adopted to meet the requirements of the railway sleepers.
In 1877, Mr. Monnier, a French gardener and inventor of reinforced concrete, suggested that cement concrete could be used for making sleepers for the railway track. Monnier in fact designed a sleeper and obtained a patent for it, but it did not work successfully. The designs were further developed and the railways of Austria and Italy produced the first concrete sleepers around the turn of the 20th century. This was closely followed by other European railways.
Much progress, however, could not be achieved till 2nd World War, when wooden sleepers practically disappeared from the markets and their prices greatly increased. Almost at the same time, as a result of extensive research carried out by the French railways and other European railways, the modern concrete sleeper was developed. Heavier rail sections and long welded rails were also being produced. This necessitated the need for a better type of sleeper, which could fit in the modern track. These conditions gave a spur to the development of concrete sleepers and countries such as France, Germany and Britain went in a big way for development of these concrete sleepers to perfection.
[edit] Plastic sleepers
In more recent times, a number of companies are selling railroad ties manufactured of recycled plastic materials. These ties are said to outlast the classical wooden tie, be practically impervious to the seasons, but otherwise exhibit the same properties as their wooden counterparts with respect to damping of impact loads, lateral stability, and sound absorption. These products have gained limited acceptance by railroads.
[edit] Fastening rails to sleepers
Sleepers are laid across the grade at intervals of about two feet. The rails are then laid atop the ties, perpendicular to them. If the ties are wood, then cleats are laid down and either spikes or bolts driven through them 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 sleeper 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 "datenail" 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. A typical mile of rail contains approximately 3,000 ties.
[edit] Other uses
In recent years, wooden railroad ties have also become very 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.
