Sleepers
Sleepers, also known as ties in the US, are one of the most recognizable railway components.
Made from different materials, each with their own pros and cons, they do so much more than just hold the rails apart. Read on to fin out more!
The Functions of a Railway Sleeper
The sleepers on the railway have a number of functions that make them, and the maintenance of them, critical to the condition of the rail way. Sleepers have five main functions:
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To distribute the load of the train over a wider area and down into the ballast below. This reduces the stresses on the underlying ballast and formation, that are caused by the loads of passing trains.
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To hold and maintains the rails the correct distance apart. This distance is know as the gauge.
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To resistance longitudinal movement of the track. This is done by their weight as well as their shape. The shape gives a large surface area
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To also prevent lateral movement. This can be caused by the forces of passing trains or thermal forces in the rails
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To electrically insulate the two rails from each other. This is important for both signalling and traction systems
What materials are sleepers made of?
The material a sleeper is made of is very important. Firstly it has to be able to allow the sleeper to perform the functions listed above. It also has to be resilient. A sleeper, once installed, has to withstand the weather all year round and loading and unloading of passing trains while maintaining its performance.
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Sleepers are made from three difference materials; wood, concrete and steel.
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Wood
Wood or timber sleepers are easy to handle and versatile, given they can be re-drilled easily. This means that they can be pulled through and re-drilled to fit a new baseplate.
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However, their relatively light weight gives less mass to the track, a disadvantage in times of hot weight. Holes drilled for baseplates are susceptible to elongation, leading to gauge issues. Even when treated, they deteriorate faster than sleepers of other materials.
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Steel
Steel sleepers are also relatively light, given that they are stamped out of sheet steel. They also have good resistance to lateral and longitudinal movement. As they are metal, they also last a lot longer than wood. They also don't have the tendency to crack like concrete.
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But their hollow design makes them hard to lift and pack to repair top faults while their spade ends prevent sluing. Also, being metal they are harder to insulate which can cause signalling problems.
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Concrete
The most common in the UK. Its weight and dimensions lead to a secure track construction that can resist thermal and other forces. They have a long life and provide good, consistent maintenance of gauge.
While their weight is an asset, it does make manual maintenance tasks, such as changing them, challenging.
New Innovation
A new material that is being used more and more for sleepers these days is plastic. It's largest positive attribute is its lower environmental impact than either wood or concrete.
Suitable hard wood for sleepers has to be harvested and shipped from where the trees are found that are of the correct age and quality.
Concrete production uses a large amount of energy and water while also releasing a lot of CO2.
Given its relatively new introduction, its long-term performance and pros and cons are still to be seen.
