Railroad tie
A railroad tie, crosstie, railway tie or railway sleeper is a rectangular support for the rails in railroad tracks. Generally laid perpendicular to the rails, ties transfer loads to the track ballast and subgrade, hold the rails upright and keep them spaced to the correct gauge.
Railroad ties are traditionally made of wood, but prestressed concrete is now also widely used, especially in Europe and Asia. Steel ties are common on secondary lines in the UK; plastic composite ties are also employed, although far less than wood or concrete. As of January 2008, the approximate market share in North America for traditional and wood ties was 91.5%, the remainder being concrete, steel, azobé and plastic composite.
Tie spacing may depend on the type of tie, traffic loads and other requirements, for example on North American mainline railroads to on London, Midland and Scottish Railway jointed track.
Rails in North America may be fastened to the tie by a railroad spike. Iron/steel baseplates screwed to the tie and secured to the rail by a proprietary fastening system such as a Vossloh or Pandrol are commonly used in Europe.
Types
Stone block
The type of railroad tie used on the predecessors of the first true 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, such as at Chat Moss, where timber ties had to be used. [|Bi-block] ties with a tie rod are somewhat similar.Wooden
Historically wooden rail ties were made by hewing with an axe, called axe ties, or sawn to achieve at least two flat sides.A variety of softwood and hardwood 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 more readily available.
Softwood is treated, with creosote being the most common preservative for railway ties. Other preservatives used include pentachlorophenol and chromated copper arsenate. Sometimes non-toxic preservatives are used, such as copper azole or micronized copper. 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 are durable enough that they can be used untreated.
Problems with wooden ties include rot, splitting, insect infestation, plate-cutting, also known as chair shuffle in the UK and spike-pull. Wooden ties can catch fire; as they age they develop cracks that allow sparks to lodge and more easily start fires.
Concrete
Concrete ties are 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 ties 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 ties need to be installed on a well-prepared subgrade with an adequate depth on free-draining ballast to perform well. It is a common misconception that concrete ties amplify wheel noise. A study done as part of Euronoise 2018 proved this false, showing concrete sleepers to be an average of 2dB quieter than wooden ones, however with a greater acoustic sharpness on straight stretches of track. Concrete ties were however shown to be quieter than wooden ties almost universal across the audible frequency band on curves. This causes train noise when over concrete ties to potentially be subjectively perceived as louder than train noise over wooden ties.On the highest categories of line in the UK, pre-stressed concrete ties 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.
Steel
Steel ties are formed from pressed steel and are trough-shaped in section. The ends of the tie are shaped to form a "spade" which increases the lateral resistance of the tie. Housings to accommodate the fastening system are welded to the upper surface of the tie. Steel ties 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 ties are lighter in weight than concrete and able to stack in compact bundles unlike timber. Steel ties can be installed onto the existing ballast, unlike concrete ties 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 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 where the dry, hot climate made wood ties unsatisfactory.
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 1s, regional, shortlines, mining, electrified passenger lines and all manner of industries. Notably, steel ties have proven themselves over the last few decades to be advantageous in turnouts and provide the solution to the ever-growing problem of long timber ties for such use.
When insulated to prevent conduction through the ties, steel ties may be used with track circuit based train detection and track integrity systems. Without insulation, steel ties may only be used on lines without block signaling and level crossings or on lines that use other forms of train detection such as axle counters.
Plastics
In more recent times, a number of companies are selling composite railroad ties manufactured from recycled plastic resins and recycled rubber. Manufacturers claim a service life longer than wooden ties with an expected lifetime in the range of 30–80 years, that the ties are impervious to rot and insect attack, and that they can be modified with a special relief on the bottom to provide additional lateral stability. In some main track applications the hybrid plastic tie has a recessed design to be completely surrounded by ballast.Aside from the environmental benefits of using recycled material, plastic ties usually replace timber ties soaked in creosote, the latter being a toxic chemical, and are theoretically recyclable. However, plastics may shed microplastics and leach other possibly toxic chemicals such as ultraviolet inhibitors.
Hybrid plastic railroad ties and composite ties are used in other rail applications such as underground mining operations, industrial zones, humid environments and densely populated areas. Hybrid railroad ties are also used to replace individual rotted wood ties, which will result in intermingled hybrid and wood ties and continuous track stiffness. Hybrid plastic ties and composite ties also offer benefits on bridges and viaducts, because they lead to better distribution of forces and reduction of vibrations into respectively bridge girders or the ballast. This is due to better damping properties of hybrid plastic ties and composite ties, which will decrease the intensity of vibrations as well as the sound production. In 2009, Network Rail announced that it would begin replacing wooden ties with recycled plastic. but I-Plas became insolvent in October 2012.
In 2012, New Zealand ordered a trial batch of "EcoTrax" brand recycled composite ties from Axion for use on turnouts and bridges, and a further three-year order in 2015, but then Axion filed for bankruptcy in December 2015, though it continues to trade. These ties are developed by Dr. Nosker at Rutgers University.
Composite sleepers, manufactured from various recycled plastics, were introduced in Wiltshire, United Kingdom, in 2021. They were installed as an alternative to wooden sleepers, on a bridge where concrete sleepers would have been too heavy. Although it was the first instance of plastic sleepers being installed on mainline track in the country, they have previously been used on narrow-gauge railways.
Ties may also be made from fiberglass.