Why Are Railway Ties Laid Under Rails

Railway ties are an essential part of the track structure. They bear various forces exerted by the rails and elastically transfer these forces to the ballast, while effectively maintaining the track gauge, direction, and alignment. The forces acting on the railway ties are extremely complex, including vertical, lateral, and longitudinal forces. Additionally, the ballast structure supporting the ties is complex, and the supporting conditions vary randomly, constantly changing the loading conditions of the ties. This complexity in load distribution dictates the intricate design requirements for railway ties.

Wooden ties, made from timber, are also known as "railroad ties" or "sleepers." Pressure-treated wooden ties are sometimes referred to as "oil-impregnated ties." These wooden ties were the first type used in railroads and are still in use today. Depending on their location on the track, wooden ties are categorized into three types: ordinary wooden ties, switch ties (also called "point ties"), and bridge ties (also known as "bridge sleepers"), as shown in Figures 1 to 3.

Advantages of Wooden Ties:
Wooden ties have good elasticity, which helps absorb and mitigate the dynamic impact forces from trains. They are easy to process and manufacture, and the connection with rails is relatively simple. They are also convenient for transportation, installation, and maintenance. Wooden ties are lightweight and provide good insulation properties. Due to the higher friction coefficient between the wooden ties and ballast, they ensure track stability.

However, wooden ties require a large amount of high-quality timber, and due to limited resources, their cost is relatively high.

Disadvantages of Wooden Ties:
Wooden ties have difficulty maintaining track geometry, are prone to mechanical damage, decay, and wear, and have a shorter service life. This can also be attributed to limitations in production processes. Additionally, the strength and elasticity of wooden ties can vary due to differences in timber species and the parts used, which can cause unevenness in the track when subjected to the forces of locomotives and vehicles, thus increasing wheel-rail dynamic forces.

By structure and placement:

Lateral ties, Longitudinal ties, Ladder-type ties, Short ties, Double-block ties, Y-type ties, 

By usage requirements:

Ordinary ties, Switch ties, Bridge ties

By material:

Wooden ties, Concrete ties, Steel ties, Composite material ties