Guys
stone blocks
Prior to the first modern railway (the Liverpool and Manchester Railway, opened in 1829), pairs of stone blocks placed on the ground were used, using nails and wooden wedges to hold the rails fixed to the blocks, as on the Kilmarnock and Troon Railway,[11] a 10-mile-long line in Scotland, which had been built in 1812 to transport coal by animal draft.[12] An advantage of this method of construction was that it allowed horses to walk between the two rails without risk of tripping.
As increasingly heavier locomotives were used, it was found difficult to maintain the correct gauge. Stone blocks were, in any case, inadequate in soft terrain such as peat bogs, where wooden sleepers had to be used. The bi-block sleepers are reminiscent of this type of supports.
Wood
Historically, wooden sleepers had the upper and lower faces sawn, to guarantee at least two flat and parallel sides, while the remaining faces were usually hewn with an axe.[13] The appearance of mechanized sawmills made it common for all faces to be sawn. With the later introduction of some types of base plates, it is common for them to be supplied with recesses to place these plates, and when threaded lag bolts are used, it is also common for them to come from the factory with holes drilled at the exact distance to place the rails.
A wide variety of woods are used, preferably hard (although not necessarily), but always of large species, with trunks capable of being cut into rectangular prisms between two and three meters in length, and with sections of about 20x30 cm. Common woods in Europe are oak, beech[13] or tropical varieties imported from Africa such as akoga.[14] In other places some types of eucalyptus are used (jarrah and karri), and the red quebracho, common in South America. These hardwoods, due to their great durability, have been widely used, although they are increasingly difficult to obtain, especially from sustainable sources.[15].
In some countries, softer woods are used, such as Douglas fir, other types of conifers or white quebracho. While they have the advantage of absorbing protective treatments more easily, they are more susceptible to wear, but they are cheaper woods, lighter (and therefore easier to handle) and are more widely available.[15].
Creosote (banned in Europe since 2002 due to its potential toxicity)[3] has been the most common preservative treatment, although sometimes compounds such as pentachlorophenol or chromated copper arsenate are also used, and sometimes non-toxic substances are used, such as micronized copper or copper azole.
New boron-based wood preservation technology is being used by major US railroads in a dual-treatment process to extend the life of wooden sleepers in humid areas.[16] Some woods (such as sal, mora "Mora (genus)", jarrah or metel) are durable enough to be used without treatment.[17].
Conservation problems for wooden sleepers include rot (caused by fungi in humid environments), splitting (due to irregular graining of the wood), insect infestation (especially xylophagous such as termites), abrasions (caused by the lateral movement of the base plates), and unusability of nailing or screwing holes (due to the tearing out of pickets or lag screws, caused by the rail torsion generated by the loaded axles).[18].
Although it is not common, wooden sleepers can catch fire, and as they age, they develop cracks that allow combustible substances such as diesel or lubricants to accumulate, which can cause sparks from moving material to ignite the wood more easily.
The main advantages of wooden sleepers are their potential availability (they can be adapted almost immediately to any track width with minimal requirements), their competitive initial installation price and their lightness (which makes them very suitable for laying temporary detours and auxiliary tracks in numerous temporary construction situations of any other type of track). At the same time, its low weight means that track maintenance operations can be carried out with simple equipment, without the need to use expensive heavy machinery. In the event of a derailment, they usually withstand the passage of dislocated wheels without catastrophic damage, and from a dielectric point of view, they offer good insulation from the ground.
As disadvantages compared to concrete sleepers, it is worth mentioning their lower durability and stability against the loads of the moving material, sometimes aggravated by the aforementioned external factors (such as rot or insect attack), and the problem of loosening of the fastenings, which requires careful monitoring of their condition, which requires intensifying maintenance tasks.
Concrete
Concrete sleepers can be monoblock or biblock. The former are made up of a single piece of prestressed concrete, while bi-block sleepers consist of two pieces of reinforced concrete joined by a steel bar (brace).
Furthermore, monobloc concrete sleepers can be multipurpose[19] if the rails can be fixed in two different positions to modify the track width, and mixed or three-wire sleepers,[20] when they allow trains of two different widths to circulate on the same track.
For years, with the appearance of different types of electrical locks, the sleeper has to be electrically isolated with respect to the rails.
Concrete sleepers, although with a higher initial installation cost than wooden sleepers, are more economical in the long term, and when specialized manufacturers are available, they can be obtained from abundant materials such as concrete and steel, and without the environmental constraints involved in obtaining top quality wooden sleepers from sustainable forestry farms.
Its manufacture is a perfectly standardized industrial process, in which strict control of the characteristics of the materials used (mainly concrete and steel) and the manufacturing processes is used, including the curing of the concrete in an autoclave. The polyethylene sheaths or spikes into which the screws are screwed are designed to be firmly embedded in the concrete.
Their greater rigidity and stability allows them to withstand higher axle loads and higher driving speeds, and their greater weight contributes decisively to maintaining the stability of the track geometry", especially when installed with continuous rails (with the so-called long welded bar technique). In addition, they have a longer useful life and require less maintenance, and the use of elastic clips to hold the rails minimizes the problem of loosening of the pickets in the wooden sleepers.
Due to their great rigidity, they must be installed on a well-prepared platform, with an adequate thickness of ballast under the underside of the sleepers to allow correct distribution of the rolling stock loads and guarantee adequate drainage. Concrete sleepers amplify wheel noise, so in some cases wooden sleepers have been used in densely populated areas.
Other lines of research propose improving the properties of concrete and steel, adding different types of fibers to the concrete, or replacing steel reinforcements (which have the problem of becoming brittle at very low temperatures) with rods of various mineral fibers compressed with polyethylene covers. Specifically, in countries with very low temperatures such as Russia, sleepers armed with basalt fiber rods, obtained from this rock, have been investigated.[23].
Steel
These sleepers are formed from pressed steel (cheaper) or cast iron (higher rust resistance) and have a ribbed section, so they can be stacked in compact packages. Its ends are designed to form a "shovel", which increases its lateral resistance. The anchors to accommodate the fixing system are welded to the surface of the upper face. Steel sleepers have been widely used in the United Kingdom, mainly on secondary or low-speed lines, and are suitable for this purpose due to their economical solution and the simplicity of their installation on a layer of ballast. Each steel sleeper weighs about 80 kg, while the cast ones reach 100 kg,[24] being much lighter than the concrete ones. They are 100% recyclable and require up to 60% less ballast than concrete sleepers, and up to 45% less than wooden sleepers.
Historically, they have not been designed efficiently, suffering the effects of increased traffic loads during their normally long useful lives. These old and often obsolete designs posed a limitation to the load capacity and speed of train circulation, but they can still be seen in many places worldwide (there are designs from several countries, and even standardized models such as the UIC 28),[24] in adequate condition despite decades of service. It is common to find steel sleepers with more than 50 years of service, and in some cases, they can and have been rehabilitated and continue to function well. They were also used in special situations, such as the Hejaz Railway in Arabia, which had an ongoing problem with Bedouins stealing wooden sleepers to make campfires.[25].
Modernly designed steel sleepers support heavy loads, and are more economical to install in new construction than creosote-treated wood sleepers or concrete sleepers. They are used in almost all sectors of the world's railway systems, and solve the growing problem of having the long wooden sleepers used in track devices; although their lightness, potential oxidation problems, and the difficulty of electrically insulating them correctly do not make them suitable for high-speed lines or heavy traffic.[24] When electrically insulated from the rails, they can be used with train detection and track integrity systems based on track circuits. Without isolation, they can only be used on lines without automatic locking or on lines that use other forms of train detection, such as axle counters.
Plastics
Since the end of the century, several companies have been marketing sleepers made from composite materials, such as recycled plastic[26] or recycled rubber agglomerated with resins. Among their advantages, mention is made of a longer useful life (between 30 and 80 years) than that of wooden sleepers, that they are immune to rot and insect attack,[27][28][29] and that they can be configured with a special support at the bottom to obtain greater lateral stability.[27] In some applications for main roads, they are designed to work completely surrounded by ballast. Despite its potential advantages, its use has not become widespread among the main railway managers, who continue to use more traditional materials such as wood and concrete.
In addition to the environmental benefits of using recycled material, plastic sleepers can advantageously replace wooden sleepers treated with creosote (a toxic chemical),[30] while being fully recyclable. Hybrid and composite plastic sleepers[27] are used in other railway applications, such as underground mining operations, industrial zones,[31] humid environments and densely populated areas. They are also used in maintenance operations to replace rotten wooden sleepers.
Likewise, their use in bridges and viaducts has some advantages, since they provide better distribution of loads and contribute to the reduction of vibrations in the bridge beams or ballast. This is due to its better damping properties, which translates into a decrease in the intensity of vibrations and noise generation.[32].
In 2009, Network Rail announced that it would begin replacing wooden sleepers with sleepers made from recycled plastics produced by I-Plas Ltd of Halifax, West Yorkshire;
In 2012, New Zealand Railways ordered a trial batch of "EcoTrax" brand composite sleepers from Axion for use on switches and bridges,[35][36] placing a further order in 2015.[37] These sleepers have been developed by Dr. Nosker at Rutgers University.[38] Some models have been made from reinforced plastic. with glass.[39].