Rail pads, originally called sole plates, are placed between the rail base and the sleeper when rail is attached to concrete sleepers instead of timber. Their main function is to reduce fatigue cracking of concrete sleepers, which is driven by impact and vibration from passing trains. Rail pads are classified along three main dimensions: material composition, installation location and design features.
Classification by Material Composition
Material selection is the most basic classification criterion because it directly controls elasticity, durability, weather resistance and load-bearing capacity.
Rubber Pads
Natural rubber offers high resilience and dynamic shock absorption, while synthetic rubber such as styrene-butadiene rubber improves wear resistance and cost effectiveness. Rubber pads suit high-speed railways, heavy-haul freight lines and urban metros where strong vibration attenuation is required. Many rubber pads have precision-engineered grooved surfaces that optimize load deflection, ensure uniform contact between rail and pad and help heat dissipation.
| Parameter | Value |
|---|---|
| Stiffness | 90 to 130 kN |
| Hardness | 72 to 80 Shore A |
| Electrical resistance | not less than 1 x 10^6 ohm |
| Tensile strength before aging | not less than 12.5 MPa |
| Elongation before aging | not less than 250 percent |
HDPE Pads
High-density polyethylene is a thermoplastic material with outstanding wear resistance, mechanical strength and resistance to chemicals, UV radiation and moisture. HDPE pads suit harsh environments such as deserts, sandy regions and areas with intense sunlight. The low friction coefficient reduces abrasion between rail and pad and extends service life in high-traffic corridors.
| Parameter | Requirement |
|---|---|
| Density | 0.95 to 0.98 g/cm3 |
| Tensile strength | not less than 19 MPa |
| Elongation | greater than 80 percent |
| Melting point | 170 to 190 deg C |
| Insulation resistance | not less than 1 x 10^10 ohm |
| Hardness | not less than 98 Shore A |
EVA Pads
EVA is a copolymer of polyethylene and vinyl acetate that balances performance, flexibility and cost. It suits low-speed rural lines and medium-traffic urban rail with moderate vibration damping, good compression recovery and easy manufacturing. Hardness can be customized between 50 and 80 Shore A.
EPDM Pads
EPDM is a synthetic rubber with excellent aging resistance, weatherability and resistance to ozone and temperature extremes from minus 50 to plus 80 deg C. It is ideal for under-sleeper applications on bridges, tunnels and coastal railways where long-term outdoor exposure demands durability.
Classification by Installation Location
Under-rail pads are installed between the rail base and the sleeper with precise stiffness to balance vibration damping and rail stability; typical thickness is 3 to 20 mm.
Under-sleeper pads are placed beneath the sleeper above the ballast to add elasticity on bridges, tunnels, viaducts and urban sections and to protect sleepers from crushing.
Under-ballast pads are laid beneath the ballast layer for comprehensive vibration isolation in tunnels, metros and lines passing through residential areas.
Classification by Design Features
Grooved pads use linear, grid or hexagonal grooves to control compression stiffness, ensure stable rail-pad contact, drain water and reduce heat build-up. Composite pads combine materials such as fiberglass-reinforced polyurethane and cork-rubber composites to balance strength, insulation and damping in demanding applications.
Selection Guidelines
Match pad stiffness to axle load and track speed
Consider climate, UV exposure and chemical environment
Verify electrical resistance where track circuits are used
Confirm pad dimensions against the rail base and sleeper seat
Frequently Asked Questions
Why are rail pads needed on concrete sleepers? They reduce impact and vibration from passing trains, which prevents fatigue cracking of the concrete sleeper.
What is the most common rail pad material? Rubber is the most widely used material because it offers high resilience and strong shock absorption.
What is the difference between under-rail and under-sleeper pads? Under-rail pads sit between the rail base and sleeper, while under-sleeper pads sit beneath the sleeper above the ballast.
Which pad suits hot desert environments? HDPE pads resist UV radiation and moisture and perform well in arid and sandy regions.
Can rail pad hardness be customized? Yes, EVA pads can be produced with hardness between 50 and 80 Shore A to match specific load and stiffness requirements.
