A rail fastening system has four jobs: hold gauge, transfer longitudinal and lateral forces from the rail into the sleeper or slab, provide controlled elasticity so that wheel-rail impact is buffered rather than passed straight into the concrete, and insulate the two rails electrically from each other in track-circuited territory. The SKL system meets these requirements with a small number of proven components:
Tension clamp (rail clip): provides the clamping force that presses the rail foot onto the pad and resists rail rotation and climb.
Screw spike or bolt: anchors the clamp assembly to a wooden sleeper, a concrete sleeper insert or a slab-mounted base plate.
Elastic rail pad: sits between rail foot and sleeper or base plate, distributing pressure and damping high-frequency vibration.
Insulating components: gauge blocks, insulators and sleeves that maintain electrical isolation.
Base plate (slab track): spreads load into the concrete slab where no sleeper is present.
Because the components work as an assembly, a change to one of them changes clamping force, pad stiffness or insulation resistance, so they are specified and inspected as a set.
Ordinary Railway Applications
On ordinary lines the governing challenge is the longitudinal and lateral force generated by train operation, together with long-term vibration and fatigue. The tension clamp and screw spike lock the rail to the sleeper, dispersing braking, traction and curve forces so that the track cannot creep or deform. A typical case is an ordinary line crossing mountainous terrain, where the alignment has large elevation changes and the train produces strong impact forces and vibration. Synergy between clamp and screw spike holds the rail firmly, while the elastic rail pad absorbs part of the impact energy and protects both rail and sleeper from abrasion. The result is a stable line with lower maintenance frequency, which matters most on sections where track access is difficult and expensive.
High-Speed Railway Requirements
High-speed operation, with train speeds reaching 350 km/h and above, raises the demand on track smoothness, stability and dimensional precision. Fasteners for this duty are manufactured with a high-precision process in which the matching tolerance between components is controlled within ±0.1 mm. During track laying, survey and measuring equipment is used to calibrate the position and accuracy of each fastening assembly so that the designed geometry is achieved across the whole section. High-elastic rail pads and high-strength tension clamps then absorb and buffer impact as the train passes, cutting vibration and noise and improving ride comfort. The same precision also reduces the probability of safety events such as derailment, because gauge and rail inclination stay within limits at speed.
Heavy-Haul Load Demands
Heavy-haul operation applies loads several times those of ordinary traffic, with axle loads and tonnage per train far above passenger service. The fastening system answers this with higher component strength and an optimised structural design rather than with a thicker rail alone.
Clamps and spikes are produced from high-strength alloy or spring steel with controlled hardness and surface quality, so that tensile strength and fatigue life remain adequate under repeated heavy loading.
Rail pads are increased in thickness and hardness to raise load-bearing capacity and wear resistance, keeping the rail-sleeper connection stable over long service intervals.
Gauge retention is checked more frequently, since small lateral movements accumulate faster under heavy traffic.
Together these measures extend track service life, reduce operating cost and secure the transport capacity that heavy-haul corridors are built to deliver.
Materials, Standards and Coating Selection
Material and coating selection determines how long the specified clamping force can be held in a corrosive and vibrating environment.
| Component | Typical material | Reference standard | Typical protection |
|---|---|---|---|
| Screw spike / bolt | Alloy steel, property class 8.8 or 10.9 | ISO 898-1 | Zinc electroplating or zinc-flake coating |
| Tension clamp | Spring steel, e.g. 60Si2MnA | GB/T 1222 | Zinc-flake coating or hot-dip galvanizing |
| Rail pad | EPDM or NR/SBR elastomer | Specified stiffness and ageing limits | Anti-ageing additive package |
| Heat-treated fastener steel | 40Cr | GB/T 3077 | Zinc plating or phosphate plus oil |
Coatings must be specified by service environment. Coastal, tunnel and de-icing-salt sections need higher corrosion protection, while dry inland lines can use a simpler coating system. Where the coating thickness changes the fit of the assembly, the threaded and bearing surfaces are masked so that clamping force is unaffected.
Installation, Inspection and Maintenance Practice
Installed performance is decided by torque control. Under-torquing leaves insufficient clamping force and allows rail movement; over-torquing yields the screw spike, strips the insert or cracks the sleeper. Good practice therefore covers the following points:
Use a calibrated torque wrench and record values by position for each section.
Check pad condition, clamp toe wear and insulator integrity while the fastener is open.
Re-tighten to specification after the first months of traffic, when initial settlement is complete.
Replace clamps that have lost toe load or show corrosion pitting at the contact point; a used clamp should not be re-installed at a location with a different rail section.
Keep the insert thread clean, and lubricate only with the medium recommended for the fastening system, since some greases attack elastomer pads.
Frequently Asked Questions
Q: What forces must a rail fastening system resist?
Vertical wheel load, lateral and longitudinal forces from traction, braking and curves, plus rail rotation and thermal movement of the rail.
Q: Why are fastener tolerances tighter on high-speed lines?
Above 350 km/h, small geometry errors produce large dynamic forces, so component matching tolerance is controlled within ±0.1 mm to keep the track smooth and stable.
Q: Which materials are used for clamps and spikes?
Spring steel such as 60Si2MnA for clamps and heat-treated alloy steels such as 40Cr for spikes, with bolt property classes typically 8.8 or 10.9 to ISO 898-1.
Q: How is corrosion protection chosen?
By environment: zinc-flake or hot-dip galvanized systems for wet, coastal or tunnel sections, and simpler zinc plating for dry inland lines.
Q: What happens if a screw spike is over-torqued?
The spike may yield or the insert or sleeper may crack, both of which remove clamping force and allow the rail to move under traffic.
Q: Why do heavy-haul lines need thicker rail pads?
Greater pad thickness and hardness raise load-bearing capacity and wear resistance, keeping the rail-to-sleeper connection stable under repeated heavy axle loads.