Railway Track System

The first topic of the Railway Engineering 101 course should undoubtedly be the track system. Knowing what components the railway line consists of, their purposes, interactions with each other, and types deeply affect everything from how to design to how to implement, so today I want to explain the railway track system in general.

The traditional railway track system is divided into two as superstructure and substructure. The cross-section is formed by the rails, rail fastening elements, sleepers, ballast, sub-ballast and subgrade layer, from top to bottom (See Figure below).

Figure 1. Rail Track System

The main purpose of the entire railway track system is to create a comfortable and safe driving environment by safely transmitting the train loads coming from the superstructure to the infrastructure. So what is the role of each component in this mission?

Rails

Rails, one of the most important elements of the railway superstructure, are the first elements that come into contact with the wheel. The main functions of the rails are to hold, guide and carry the weight of the trains on the line. Considering the weight of heavy-duty trains up to 35 tons and the speed of high-speed trains up to 350 km/h, the importance of rail design increases. In addition to the design weight and speed, the forces that the train will apply to the rails on the curves, the internal forces resulting from the extension under high temperatures and contraction under low temperatures in the rails, and the forces that will be transferred to the rails due to the acceleration and braking movements of the trains are important factors in the evaluation of rails. Since rails are expected to be resistant to abrasion and have high strength, they are manufactured from high-carbon steel. Rails are brought to the site in different lengths according to the codes in the countries and the requirements of the project and are welded on-site at various distances. Rails with different characteristics are used in light rail systems such as metro and tram, conventional and high-speed trains, switch areas and level crossings. I plan to talk about the types of rails, production procedures, testing procedures and features requested in the specifications in detail in another article, so we can just touch on the surface for now and move on.

Sleepers

Sleepers -designed in different sizes from different materials such as wood, reinforced concrete, and steel- absorb the vertical, horizontal, and longitudinal forces coming from the rail sets and transfer them to the ballast layer. Moreover, they play a key role in the continuity of the line by keeping the gauge between the rails constant. In addition, sleepers partially dampen the noise and vibrations that occur after the wheel-rail interaction and reduce the positive impact on the environment.

When the historical development of the railway is examined, it is seen that sleepers were first manufactured from wood. Since the wood material is affected too much by natural environmental conditions and external effects and this situation requires maintenance, there was a transition to steel and concrete sleepers. The use of trains with high axle loads has led to the need to make sleepers more rigid. Concrete sleepers are used in most of the train lines built today. However, the type of sleeper to be used within the scope of the project, sleeper dimensions and spacing are determined by considering the types of trains that will pass on the line, the traffic density of the line and the environmental conditions. I also plan to talk about the types of sleepers, production procedures, testing procedures and features requested in the specifications in detail in another article.

Rail Fastening Elements

When I talked about rails, it was mentioned that one of their most important tasks is to transfer the axle loads coming from the superstructure to the sleepers. In the railway track system, this load transfer is carried out by rail fastening elements consisting of components such as tension clamps, insulation parts, and intermediate baseplates. The first component that receives and transfers not only the axle loads but also the internal forces occurring within the rails is the rail fastening material. In addition, it has important effects in keeping the tension constant and increasing the elasticity of the line.

Although rail fastening materials play a very important role in the track system, there are not yet sufficient approaches for their design and analysis. More often, the qualification tests of the components that make up the fastening material are carried out.

Ballast and Sub-ballast Layer

The ballast and sub-ballast layer, called the granular layer, absorbs the loads coming from the railway superstructure and transmits them to the subgrade layer by damping them in a way that does not cause any deformation in here. This layer, in which the sleepers "float", has the functions of ensuring the drainage and elasticity of the line, keeping the sleepers fixed in all directions and protecting the platform.

The physical properties and minimum requirements of the materials forming the ballast and sub-ballast layer are different from each other. The main differences are their grain diameter, gradation, resistance to various physical conditions, and implementation procedure. Various methods have been developed in different regulations for the calculation of the granular layer thickness. I will give a detailed explanation about these methods, the properties of the materials forming the granular layer, the experiments carried out, the laying methods, and maintenance works in another article.

Ballastless Track

When it comes to railway track systems, ballasted cross-section comes to mind directly, but ballastless track or slab track stands out as a very frequently used method. So what are the benefits of this method? Although the initial cost is high, the fact that the maintenance frequency is rare and the maintenance cost is low provides a great advantage over the ballasted line. In addition, if a new project is being carried out, slab track can be preferred because it allows higher superelevation and lower curve radii. Although it is a low criterion in terms of being taken into consideration, the fact that there is no risk of ballast pieces flying out during train passage is also a positive effect for slab track.

During train passages on ballasted lines, rising sleepers fall on the ballasts and cause mechanical breakage and deterioration. In addition, due to the need for correction of ballasts over time (tamping), the inability to easily use the machines that will perform this correction in bridges and tunnels, and the fact that ballast, which is a special material, cannot be easily obtained, many sections have switched to the slab track method.

Slab track design is a completely different component according to the test procedures and specification rules that must be followed. I will explain this in detail in another article.

The basic components that consist the railway track system have been described above at a 101 level. Of course, each component has very detailed technical specifications, construction methods, testing procedures, and application areas. All of these will be explained specifically in other articles. However, the point I want to emphasize here is that the main purpose of the railway track system is to protect the integrity of the line by transmitting repetitive and dynamically effective train loads to a sub-component in a safe and comfortable way. Each component should be checked periodically and deformations that will disrupt this task should be prevented. Each component of this system, which works in harmony, closely affects the other. For example, the bending moments that will occur in the sleeper vary depending on the degree of compression and quality of the ballast layer. Or the elasticity of the rail connection material changes the effects on the sleeper. It is important to understand the track system by considering these effects.

Arifcan Yilmaz, MSc

Civil Engineer

Bibliography

1. Track Compendium, Dr. Bernhard Lichtberger

2. New Advances in Analysis and Design of Railway Track System, J. Sadehgi