13.1 Wooden sleepers
Figure 13.1 Wooden sleeper
A variant fastening of rails to wooden ties .
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.
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13.2 Concrete sleepers
Figure 13.2 Concrete sleeper
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 sub grade 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
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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.
13.3 Steel sleepers
Figure 13.3 Steel sleeper
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, short lines, mining, electrified passenger lines (OHLE) and all manner of industries.
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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.
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