Rail pads, originally called sole plates, are resilient polyurethane mats assembled between the steel rail and the sleeper to protect the top of the sleeper from abrasion and impact. Rail pads are designed to have a low initial stiffness, so that the relative deformation caused by the toe load of the spring clip is significantly larger. This mechanism keeps the rail pad in intimate contact with the rail, even as the rail moves vertically.

Functions of rail pads

Rail pads reduce fatigue cracks in concrete sleepers that are believed to be caused by the impact and vibration of passing trains. In addition, the placement of rail pads under the rails achieves the following:

1. Distribution of load over a wider surface

2. Elimination of load concentration and associated fatigue stresses

3. Concentration of load on the supporting elements

4. Absorption of uneven contact surfaces between rail and supports

5. Reduction of noise and vibration

6. Sealing between the bottom of the rail and the top of the support

7. Reduction of wear on the steel rail and its supports

Rubber Pads

Common materials used in the manufacturing of rail pads include natural rubber, synthetic rubber, and composite materials. Natural rubber offers excellent durability and resilience, while synthetic rubber provides superior resistance to aging and weathering. Composite materials, such as fiberglass, provide a high strength-to-weight ratio and are highly resistant to impact and fatigue.

In the railway system, rail pads act in two main ways: rail pads can diminish shock or impact loads by providing compliance, and they reduce (by isolation and perhaps by damping) resonant vibrations excited in the concrete sleepers (lowest resonant frequency about 200 Hz) and accommodate vibration deflections in the rail sections between the sleepers (about 0.12mm amplitude, 1000 Hz). In addition rail pads prevent rail sleeper abrasion, and crushing of the sleeper under the rail foot.