Railway track structure
Railway Track Structure
As an integral engineering structure, railway tracks are laid on the roadbed and play a guiding role in the operation of trains.
Track is a component of railway lines, including rails, sleepers, connecting parts, ballast, anti-climbing equipment and turnouts. Track is an integral engineering structure that guides the operation of trains and directly bears the huge pressure of locomotives and their loads. Under the dynamic action of train operation, its various components must have sufficient strength and stability to ensure that the train runs safely, smoothly and uninterruptedly at the specified speed.
Ballasted track 1–Rails 2–Ballast 3–Sleepers 4–Fasteners
Ballastless track
Rails
The function of rails is to directly bear the weight of the train and its load transmitted by the wheels, and to guide the direction of the train. For example, a train consisting of 60 60-ton open wagons has a combined load and deadweight of about 5,000 tons. If it is a heavy-load train of 10,000 tons or even hundreds of thousands of tons, it will be even heavier. Such a huge pressure first falls on the shoulders of the rails, so it can be seen that the rails must have sufficient strength, stability and wear resistance.
steel rail structure
The cross-section of the rail is an I-shaped rail, which consists of three parts: the rail head, the rail waist, and the rail bottom. This seemingly simple I-shaped rail has the advantages of good force bearing, material saving, and the best bending resistance.
The type and strength of rails are expressed in kg/m. The heavier the mass of each meter of rail, the greater the load it can bear. The rails of the world’s first railway were 18kg/m, and the heaviest rails are in the United States, weighing 77kg/m.
The standard lengths of steel rails in my country are 12.5m and 25m. There are also shortened rails for inner tracks of curves. The shortened rails of the 12.5m standard rail series are available in three types: 40mm, 80mm, and 120mm; the shortened rails of the 25m rails are available in three types: 40mm, 80mm, and 160mm.
Rails on the track are connected by clamps and bolts, which is called rail joints. The wheel-rail dynamic effect at the joint is large, and the maintenance and repair workload is large. The joint is one of the weak links in the track structure.
The connection form of the joint can be divided into two types according to its position relative to the sleeper: suspended type and pad type. According to the relative position of the two rail joints, it can be divided into two types: relative type and staggered type. my country generally adopts the relative suspended type, where the two rail joints are aligned left and right and located between the two joint sleepers.
Sleepers
Sleepers bear the directional pressure from the rails and elastically transmit it to the roadbed, while effectively maintaining the geometric shape of the track, especially the gauge and direction. Sleepers should have the necessary firmness, elasticity and durability, and be able to easily fix the rails and resist longitudinal and lateral displacement.
The sleepers were originally made of wood, which has good elasticity and insulation, is less affected by the temperature changes of the surrounding medium, is light, easy to process and replace on the line, and has sufficient displacement resistance. The service life of wooden sleepers treated with anti-corrosion is also greatly extended, generally around 15 years.
rail sleepers
With the reduction of forest resources and the increase of people’s environmental awareness, and of course due to the development of science and technology, some countries began to produce steel sleepers and reinforced concrete sleepers to replace wooden sleepers in the early 20th century. However, because steel sleepers consume too much metal, are expensive, and are relatively heavy, they have not been widely used. Only a few countries such as Germany are still using them. Since the 1950s, many countries have begun to generally produce reinforced concrete sleepers.
Reinforced concrete sleepers have a long service life, high stability, low maintenance workload, and much lower damage and scrap rates than wooden sleepers. On seamless lines, reinforced concrete sleepers are 15-20% more stable than wooden sleepers on average, so they are particularly suitable for high-speed passenger lines.
Reinforced concrete sleepers
The length of sleepers varies depending on the application scope. In my country, the length of ordinary sleepers is 2.5m, and the length of turnout sleepers and bridge sleepers used on steel bridges ranges from 2.6 to 4.85m. The number of sleepers laid per kilometer of line is determined based on operating conditions such as railway transportation volume and driving speed, and is generally between 1520 and 1840. Generally speaking, the more sleepers there are, the greater the track strength.
The sleeper spacing is related to the number of sleepers configured per kilometer. The number of sleepers should be determined based on the traffic volume, driving speed and line equipment conditions, and should be comprehensively considered with the rails and ballast, and reasonably matched to ensure that the track has sufficient strength and stability under the most economical conditions. The denser the sleepers, the smaller the stress on the ballast, roadbed, rails and sleepers themselves. At the same time, it makes it easier to maintain the track gauge and direction, which is especially important for areas with high driving speeds. But it cannot be too dense, which is not economical, and too small a clear distance will also affect the tamping quality to a certain extent.
Concrete wide sleeper is a prefabricated concrete slab, which is similar in appearance to concrete sleeper, also known as sleeper slab. Its manufacturing process is basically the same as that of concrete sleeper. The length of wide sleeper is the same as that of ordinary concrete sleeper, both are 2.5m, while the width is about twice that of the latter. Due to its large width, wide sleeper is directly laid on the pre-compacted ballast bed surface, and the control requirements for its thickness are stricter during manufacturing.
Concrete wide sleepers are laid closely on the ballast bed, with 1,760 per kilometer. A pair of fasteners are installed on each sleeper, and the force transmitted by the rails is in a symmetrical position of the wide sleeper axis to avoid load eccentricity. Due to its large width, the wide sleeper has bending moments in both the longitudinal and transverse directions, and is a plate supported on an elastic foundation.
Compared with ordinary sleepers, wide concrete sleepers have the following advantages: wide sleepers are 55cm wide and have a bearing area twice as large as that of concrete sleepers, which greatly reduces the stress of the ballast bed. At the same time, the mass of each wide sleeper is about 500kg, which can reduce the vibration acceleration of the ballast bed, reduce the deformation of the ballast bed, delay the accumulation of residual deformation, make the track geometry easier to maintain, and strengthen the entire track structure.
The frictional resistance on the contact surface between the sleeper and the ballast bed increases, which improves the lateral stability of the track. The ballast bed resistance increases by about 80%, which is conducive to the laying of seamless lines.
Wide sleepers are laid closely together, and the gaps between the sleepers are sealed with asphalt concrete, covering the entire top surface of the ballast bed to prevent rainwater and dirt from invading the interior of the ballast bed, thereby effectively keeping the ballast bed clean and extending the cleaning cycle of the ballast bed.
The maintenance workload of wide sleeper track is very small, only 1/2 to 1/4 of that of concrete sleeper track, which reduces and improves the maintenance working conditions, reduces the number of operations, and saves maintenance costs. In addition, since maintenance work can basically be carried out on both sides of the track, it has less interference with driving and is more suitable for use on railways with busy transportation.
Fasteners
The connection between rails and sleepers is achieved through intermediate connecting parts. Intermediate connecting parts are also called fasteners. Fasteners must have sufficient strength, durability and certain elasticity to effectively maintain the reliable connection between rails and sleepers for a long time, prevent the movement of rails relative to sleepers, and give full play to their buffering and shock-absorbing performance under dynamic action to delay the accumulation of residual deformation of the track. In addition, fasteners should also be simple in structure and easy to install, disassemble, maintain and repair. The connecting parts include wooden sleeper fasteners and concrete sleeper fasteners.
rail clips
There are two main types of wooden sleeper fasteners: split type and mixed type. Split type fasteners are also called “K” type split type fasteners. This type of fastener has a large clamping force and can effectively prevent rail creep. Its disadvantages are that it has many parts, a large amount of steel, and it is difficult to replace rails. Split type fasteners are mainly used on ballastless bridges.
The hybrid fastener parts include spikes and five-hole double-shoulder iron pads. In addition to using spikes to fasten the rails, pads and sleepers together, it also uses spikes to fasten the pads and sleepers separately. This fastening method can reduce the vibration of the pads, and has fewer parts and is easy to install. Its disadvantage is that the fastening pressure is small, and when the rails are deflected upward under load, the spikes are easily pulled up, reducing the fastening pressure.
3124 rail clips
Concrete sleeper fasteners must have sufficient buckling pressure, appropriate elasticity, a certain gauge and horizontal adjustment, and insulation performance. my country’s concrete sleeper fasteners mainly use buckle plate type and spring bar type fasteners. Buckle plate type fasteners have simple parts and are more convenient to adjust the gauge, but have low elasticity and buckling pressure, and are easy to loosen during use. They are suitable for lines with rails of 50kg/m and below. The spring bar type uses spring bars to press the rails, and has the characteristics of large buckling pressure and elasticity.
