Track pads come in two different positions with two different jobs. Under-rail pads sit between the rail foot and the sleeper (or baseplate) and set the vertical stiffness of the fastening system. Under-sleeper pads (USP) sit between the sleeper and the ballast and soften the whole sleeper support. Confusing the two is a common specification error: the stiffness values, test methods and standards are different. This article separates the two families and explains how to match pad material and performance to the line type.
1. Under-Rail Pads versus Under-Sleeper Pads
An under-rail pad is part of the fastening system: its static stiffness, commonly 20-60 kN/mm for mainline fastenings, is verified in the fastening system tests of EN 13146, and the fastening system itself must meet the performance requirements of EN 13481-2 (fastenings on concrete sleepers in ballasted track) for the target category. An under-sleeper pad is bonded or cast to the sleeper bottom; its stiffness is much lower than an under-rail pad and is expressed as a bedding modulus per unit area, with the sleepers themselves covered by EN 16730. When a specification says track pad, confirm which one is meant before comparing numbers.
2. Rubber versus Polyurethane Pads
| Property | Rubber | Polyurethane |
| Elastic recovery | Excellent, absorbs high-frequency vibration | Good, denser structure |
| Hardness and wear | Moderate | Higher hardness, better wear resistance |
| Temperature range | Prone to ageing and hardening under UV and extreme heat | Stable mechanical properties from -40 to 80 degrees C |
| Compression set | Higher, deforms under long-term load | Small deformation under heavy loads; typically no more than 15% compression set after two million fatigue cycles |
| Best fit | Metro and general track where damping is priority | High-speed, heavy-load and climatic extremes |
3. High-Speed and Urban Rail Requirements
High-speed trains generate a high frequency of wheel-rail impacts, so the pad must hold its stiffness and thickness for decades. Polyurethane pads resist wear, keep stable performance across climates, and pass fatigue requirements with a compression set of no more than 15% after two million cycles. On ballastless track the dimensional stability matters most: polyurethane has low shrinkage, so it avoids track geometry deviation, reduces maintenance frequency and lowers long-term operating cost. The fastening system on concrete sleepers is verified to EN 13481-2 for the relevant speed and axle-load category.
Metro lines run through residential areas, so vibration and structure-borne noise control come first: high-elasticity rubber pads are the standard choice to cut the transmission of running noise. Tunnel environments are humid, so the pad must resist moisture and keep its insulation function to limit stray-current corrosion and electrical interference. Frequent starts and stops mean repeated impact loading, and the fatigue performance of the fastening system must meet the EN 13481-2 category appropriate to the line. Where curve radii are small, the pad and the surrounding fastening components must also hold lateral stability to prevent rail displacement, and compact pad designs are used where installation space in tunnels and on bridges is limited.
4. Matching Static Stiffness to Axle Load
Static stiffness controls the trade-off between vibration reduction and load-bearing capacity: lower stiffness gives better damping but weaker support, higher stiffness protects the rail seat but passes more impact to the structure. As a design starting point, high-speed under-rail pads are usually specified around 30 plus or minus 10 kN/mm, metro pads in the 10-30 kN/mm band, and heavier or freight-oriented fastenings use higher values. The final value must match the axle load and the fastening system tests, and the pads installed on one line must come from one batch with consistent stiffness: mixed stiffness across a line causes uneven track stress and localised wear.
5. Alpine and Cold-Region Selection
In alpine regions the pad must keep elasticity at very low temperatures. Polyurethane pads maintain stable performance down to -40 degrees C and are the preferred material; ordinary rubber hardens and cracks in severe cold. Besides low-temperature resistance, the pad must survive freeze-thaw cycles without structural damage, which is verified by low-temperature fatigue testing. Modified rubber formulations with adjusted compounding are an alternative where a rubber pad is required but the climate is cold, balancing damping with environmental adaptability.
FAQ
Q1: What is the difference between an under-rail pad and an under-sleeper pad?
An under-rail pad sits between the rail foot and the sleeper and sets the fastening stiffness, typically tens of kN/mm. An under-sleeper pad sits under the sleeper on the ballast side and softens the sleeper support at a much lower stiffness level defined per unit area. They are tested to different standards and must not be compared directly.
Q2: Why do high-speed railways prefer polyurethane pads?
Polyurethane combines wear resistance, dimensional stability and fatigue life: low shrinkage keeps ballastless track geometry, and a compression set of no more than 15% after two million cycles keeps the pad elastic for decades, cutting maintenance frequency.
Q3: Which pad material is best for metro vibration control?
High-elasticity rubber pads give the strongest vibration and noise reduction and are the standard choice for urban rail, provided the material grade is selected for the humid tunnel environment and the fatigue performance of the fastening system is verified for the line category.
Q4: What static stiffness should a high-speed rail pad have?
As a design starting point, around 30 plus or minus 10 kN/mm for high-speed under-rail pads, with the exact value confirmed by the fastening system tests for the target speed and axle load. Metro pads are typically softer, in the 10-30 kN/mm band.
Q5: Can rubber pads be used in alpine regions?
Standard rubber hardens and cracks in severe cold. Use polyurethane pads, which stay stable to -40 degrees C, or modified rubber formulations with verified low-temperature performance and freeze-thaw resistance.
Q6: Which standard covers under-sleeper pads?
Sleepers with under-sleeper pads are covered by EN 16730, while the fastening system on concrete sleepers in ballasted track is covered by EN 13481-2 with the test methods of the EN 13146 series.