The railway track is a British express of the tracks on a railway. It is also called railroad track in the United States. Usually, people talk about the railway track as a whole structure including rails, sleepers, fasteners, ballast (or slab track ) and subgrade. The structure can be divided into two parts, the superstructure and the subgrade.
The superstructure is the part above subgrade that you can see it with your eyes. Its components include rails, sleepers, fasteners, and ballast. Typical classification for the superstructure is ballasted track and ballastless track, or jointed track (tracks with gaps) and welded track (seamless track).
A ballasted track refers to a track with ballast (usually made of crushed stones) under the track. It is one of the main forms of track structure. It has the advantages of good flexibility, low price, convenient replacement and maintenance, and good noise absorption. However, compared with the ballastless track, it also has the disadvantages that the line plane geometry is not easy to maintain, most of them are short, and the maintenance workload is large.
The bottom of the track is ballast, which provides flexibility and drainage for the track. Railway sleepers are embedded in ballast and are generally laid laterally. They are made of wood, reinforced concrete or steel. The original rails were cast-iron rails and then developed into I-shaped steel rails. They are laid on sleepers and fixed with other track material.
A ballastless track is a track structure that uses concrete, asphalt mixture and other integral foundations to replace the loose gravel trackbed. The sleepers of the ballastless track are made of concrete, and the roadbed does not need gravel. The steel rails and sleepers are directly laid on the concrete road.
Ballastless track is an advanced track technology in the world today. The advantages of ballastless track over the ballasted track are in several aspects.
- Low maintenance costs.
- Slow track deformation and good durability bring long service life, good line condition.
- There will be no stone ballast splashing when driving at high speed.
- It is more comfortable and the train speed can reach more than 500 kilometers per hour.
The jointed track is a type of track structure using bolted rail joints (joint bars) to connect rails end to end. The jointed track was originally used because modern technology cannot provide any alternatives. The defects at the joints always happen because the discontinuous and uneven rail surface which lead to the additional load. As a result, it greatly affects the smooth driving and even causes geometry deformation and rail broken. Extensive maintenance work needs to be done to avoid the damages.
Welded track or seamless railway track is not completely free of gaps, but the number of gaps in the entire railway section is so small that it is almost negligible. It is a concept comparing to jointed track. Generally, it is formed by welding multiple short rails in sequence. When a rolling stock runs over, the adjacent steel rails are stressed at the same time, so that the wheels can pass through the joints of the two steel rails smoothly without vibration. The welded track has the following advantages.
Reduce noise pollution. The ultra-long seamless track is composed of many standard steel rails connected to long rails, and the length is generally two to three kilometers.
Abrasion is greatly reduced. Compared with ordinary rails, seamless rails eliminate a large number of rail joints. The direct effects are eliminating the impact of joints, reducing line damage, saving a lot of raw materials. The line maintenance is estimated can save 30%-75% of costs.
Improve the reliability of the track, and the speed of the train will increase accordingly.
Increase the stability and comfort of passengers.
The subgrade is the fundamental structure for railway tracks. Under the ballast is the part called subgrade. The railway subgrade is a structure that bears and transmits the gravity of the track and the dynamic action of the train. It is the foundation of the track and an important building to ensure the operation of the train.
The composition of subgrade
- Main body
The main body includes embankments filled with natural soil and stone, and road cuts dug in the natural stratum.
- Drainage facility
Ground drainage: Side ditch, drain, gutter
The surface water that may be stagnated within the range of the subgrade is quickly drained out, and the outside surface water is prevented from flowing into the subgrade range, so as not to seep and wet the basic body of the subgrade or form overflow to wash the subgrade slope.
Underground drainage: drainage channel, seepage ditch, seepage well.
According to hydrological or geological conditions, it is built to a certain depth below the ground to intercept, drain, draw out groundwater or lower the groundwater level, to keep the roadbed and slopes dry and improve the stability of the soil.
- Protection and reinforcement facility
- Slop protection equipement.
Paving turf, spraying, plastering, pounding, retaining walls, slope protection is used to protect soil slopes that are susceptible to damage by natural action and slope deformation. Intercepting and shielding structures built to protect against falling rocks, such as shed holes.
- Scour protection equipment.
It is used to protect the subgrade from scouring by water or waves, such as turf, riprap, masonry slope protection, retaining walls, dams, dams, etc.
- Support reinforcement equipment.
Retaining wall or pillar used to support and stabilize the subgrade.
- Anti-sand, Anti snow equipment.
Fences and protection forests are used to prevent sand, wind and snow from burying the subgrade.
The feature of subgrade
- The subgrade is a kind of soil and rock structure located in various topography, geology, hydrology and climate environment.
- Subgrade engineering is completely exposed to nature, and its engineering nature is very sensitive to changes in natural conditions and has poor resistance
- The subgrade is subjected to static load and dynamic load from the track.