What is a railroad tie?

When railway tracks were first invented, longitudinal ties were used. The system that is not adopted in the end must have its unreasonableness. The main disadvantages of this longitudinal sleeper are

1. It is easy to accumulate water between the two longitudinal sleepers, which will reduce the strength of the roadbed and aggravate the decay of the wooden sleepers;
2. There is not a large enough contact area between the sleeper and the ballast to disperse the stress;
3. The bending (warp) of the sleeper due to moisture, temperature changes, etc. will lead to the change of the gauge. Therefore, this structure was gradually replaced by the now-common cross ties.

The material of sleepers can be roughly divided into wooden sleepers (wood railroad ties), concrete sleepers (concrete ties), composite sleepers (composite ties), and steel sleepers (steel ties).

Wood railroad ties

wood railroad tie

Wooden railroad tie used to be the most widely used sleeper material, and now they have been largely replaced by concrete sleepers in China, but they are still used in some branch lines, stations, and turnout sections; in the United States, wooden sleepers are still the most important. In the form of sleepers, there are even some railway companies (such as NS) that do not use sleepers of other materials at all.

The wood of wooden railroad tie is not chosen randomly and must meet certain strength and hardness requirements, which I will not list them one by one. In the United States, the most widely used wood is oak. It is not just that any oak tree can be used as a sleeper after being cut down. The tree must meet certain age and size requirements, and it must also check the possible existence of Decay, holes, knots, shakes, splits, checks, the slope of grain, and Whether the defects such as pockets are beyond the allowable range (I am really unable to translate, everyone knows that there are many defects to check). Subsequently, the wood is cut to a specific size, dried (Seasoning), and then treated with medicine (treatment). Common drug treatments include Borate/Creosote Dual-Treatment, Zinc Oxides (Ties treated in this way are called Green Ties), Copper Naphthenate (CuNap), Ammoniacal Copper Zinc Arsenate (ACZA), Disodium Octaborate Tetrahydrate (DOT), Pentachlorophenol, etc. At present, creosote is the most widely used treatment method in the United States. At the same time, CuNap and ACZA have also passed the environmental assessment of the EPA (yes, the EPA that disclosed some time ago that some cars of the Volkswagen Group did not meet the standards, the full name is the US Environmental Protection Agency).

In fact, the way of handling wooden sleepers today has little harm to the environment (however, when changing sleepers manually, it is still forbidden to touch the sleepers with bare hands, at least it is not recommended, even if you wear gloves because you are tired. You may use gloves to wipe sweat, and special tools should be used to replace sleepers). After that, the sleeper is dried at high temperature and low pressure, then heated in a vacuum to impregnate the wood with the medicine, and finally pressurized and heated to form a protective layer of medicine on the surface. In this way, wooden sleepers are produced.

In addition, the ends of wooden sleepers are often nailed with Nail Plates or Anti-Splitting Irons, or covered with Iron Rings (Anti-Splitting Bands) to prevent cracking, see the picture below, the picture was taken by Monticello Railroad Museum. In the case of road studs that need to be pulled out, the holes left by the removal of the road studs need to be filled with formulated glue to prevent the water from rotting.

Concrete railroad tie

concrete railroad tie

For concrete railroad tie, it can be roughly divided into ordinary concrete sleepers and prestressed concrete sleepers. Ordinary concrete sleepers are concrete strips reinforced with steel structures in the middle. Concrete has a characteristic that its allowable compressive stress is significantly higher than its allowable tensile stress. Therefore, after a long period of use, the bottom of the concrete sleeper is prone to cracking caused by transverse stretching.

To overcome this problem, prestressed concrete ties were created. During the production of this sleeper, certain compressive stress is applied to the sleeper through the internal steel structure. In this way, in actual use, the transverse tensile stress can be reduced, thereby preventing cracking of the bottom of the concrete sleeper to a certain extent. Commonly used production methods for concrete sleepers include the longline system, frame system, etc. (Chinese terms may be inaccurate, please correct me if there are any mistakes).

Under the production line cutting production method, the factory will prefabricate long concrete strips, and then cut them to the required size; in the mold casting production method, the factory will directly cast the sleeper in the mold, so no cutting is required. In actual production, the quality and efficiency of these two methods are basically the same.

Synthetic railroad tie

It has similar properties to wooden sleepers and is not described in particular since they are not widely used. The picture below is a picture of synthetic material sleepers in Wikipedia. This is a relatively dazzling one. Most synthetic sleepers look similar to wooden sleepers.

Steel railroad tie

steel railroad ties

The application of steel sleepers is not particularly extensive. The lower part of the steel sleeper is hollow, and the lower part of the sleeper needs to be filled with ballast when in use. As you can imagine, this construction operation is not very convenient.
For the advantages and disadvantages of concrete sleepers and wooden sleepers, a simple comparison is as follows:

Concrete sleepers can better maintain the gauge;

Concrete railroad tie can bear a larger load (of course, this cannot be viewed independently, the load capacity of the line is closely related to each component, and is also closely related to operating indicators such as running speed);
Wooden railroad tie have better-insulating properties;
The material of wooden sleepers is soft, so the loss of fasteners and ballast is small;
As for environmental protection, this is a topic that has been debated for a long time. In fact, deforestation is not a big problem, because today’s wood sources are all from large-scale forest farms. As long as there is good operation and management, the wood will grow longer after being cut. Come out; the impact of wooden sleepers on the environment lies in the harm caused by its drug treatment, and cement production is also a high-energy-consuming and high-polluting industry, so it is actually difficult to compare which is better.

In addition, there are several special sleeper forms. Because it is not in terms of sleeper materials, it is discussed separately.

Let’s talk about Bridge Ties first. Nowadays, bridge structures in China generally have ballast beds or concrete monolithic ballast beds. However, in many countries, such as the United States, there are a large number of construction methods that directly mount the sleepers on the steel structure of the bridge. However, there will be a series of problems: first, the steel structure of the bridge is very “hard,” so the loss of fasteners will increase, and the connection with the soil subgrade outside the bridge will cause various problems due to changes in the subgrade strength.

Moreover, the steel structure of the bridge is not “flat,” so sleepers need to be used to find “flat”; in addition, without the existence of ballast, the pressure of the sleepers will directly act on the steel structure of the bridge. So, in general, bridge sleepers are thicker and more densely laid, and no two bridge sleepers are exactly the same.

Then, let’s talk about Switch Ties. In the turnout section, long sleepers need to be used because lines and equipment such as frogs and switch machines need to be laid.

Generally, the length of wooden sleepers changes in steps, while the length of concrete sleepers changes continuously. The picture below is of a wooden turnout sleeper, taken at BNSF Northern Yard, Minneapolis. In addition, some sections using the third rail also require the use of long sleepers