Anatomy of a Modern Railway Fastener System
A railway fastener system holds the rail to the sleeper or slab, maintains the gauge, transfers vertical and lateral wheel forces into the track structure, and electrically insulates the rail from the ground. A complete system therefore contains clamp or spring elements, threaded fasteners, resilient pads, insulating components and a base or shoulder detail. Five families dominate modern track: SKL, KPO, the type N spring bar system, WJ-8 and the E-type clip. They differ in how clamping force is generated, in the components used, and in the traffic and track form each is designed for.
SKL and KPO: Bolted Clamp Systems for Mixed and Heavy Traffic
The SKL system is built around a bent spring bar. It has three common variants, SKL 3, SKL 12 and SKL 14, and a single unit typically consists of two SKL tension clips, two SS35 screw spikes, two flat washers, two plastic dowels, two guide plates and one rail pad. The spring bar supplies the clamping pressure, the SS35 screw spikes fix the assembly to the sleeper, the washers spread the load, the plastic dowels insulate and position the screws inside a concrete sleeper, the guide plates locate the rail accurately, and the rail pad buffers impact and reduces wear. Clips are made from spring steels such as 60Si2CrA and 38Si7 at a hardness of about 42-47 HRC, with references to DIN 17221 and GB/T 1222 for the spring steel, ISO 898-1, UIC 864-1 and NF F 500-50 for the threaded fasteners, and EN 10025 and EN 10139 for the washers. The system is widely used on European railways and performs well on ordinary lines and heavy-haul branches.
| Component | Quantity per unit | Typical material or specification |
|---|---|---|
| SKL tension clip | 2 | 60Si2CrA, 38Si7; hardness 42-47 HRC; DIN 17221, GB/T 1222 |
| SS35 screw spike | 2 | M24 x 150 mm; grades 4.6, 5.6 and 8.8 in Q235 or 35# and 45# steel |
| Flat washer | 2 | Uls 7, Q235; EN 10025, EN 10139 |
| Plastic dowel | 2 | HDPE or reinforced nylon 66 for concrete sleepers |
| Guide plate | 2 | Reinforced nylon 66; UIC 54, UIC 60, 50 kg and 60 kg rail |
| Rail pad | 1 | EVA or rubber, size as ordered |
The KPO system uses a different principle: a clamp grips the rail foot directly instead of pressing it through a spring bar. A unit comprises two KPO clamps, fastening bolts, rail pads and insulating components. The clamp resists rail movement, the bolts set the clamping pressure, the pad absorbs shock and reduces noise, and the insulation prevents stray currents from damaging components through electrochemical action. Clips are commonly made from Q235 to DIN 5906, the base plate from cast iron QT500-7 or QT450-10 to UIC 864-6 in forged low-carbon or high-carbon steel, and the associated hex nuts and double coil spring washers from 35# or 45# steel and 65Mn respectively. Easy to install and dismantle, KPO is widely used on urban rail transit and industrial dedicated lines where frequent start-stop duty and occasional line adjustment are normal.
Type N Clip and WJ-8: Gauge Control on High-Speed and Ballastless Track
The type N spring bar system combines an n-shaped spring bar with embedded iron seats, insulated gauge blocks and rubber pads. The spring bar generates a balanced buckling force that holds gauge accurately, the embedded seat anchors the assembly in the sleeper, the insulated gauge block both adjusts gauge and provides electrical insulation, and the rubber pad damps vibration. Clips are made from 60Si2MnA or 60Si2CrA at 4 mm or 4.5 mm thickness, hardened to 42-47 HRC and fatigue tested to three million cycles without fracture, while insulators are moulded from glass-fibre reinforced nylon 66. The system suits international railway projects and lines where track stability and consistent fastener elasticity are critical, including high-speed construction.
WJ-8 is a shoulder-type system developed for high-speed ballastless track. Components include spiral spikes, flat washers, spring bars, insulated gauge blocks, gauge baffles, rail pads, iron pads, elastic pads under the iron pads and embedded sleeves, plus fine-tuning pads for height adjustment. Spring bars are supplied as the W1 type with a 14 mm diameter for general sections and the X2 type with a 13 mm diameter for bridge sections; rail pads are rubber on general sections and composite on bridges. Insulating blocks are made in type I for general sections and type II for rail joints, and spiral spikes in S2 form for height adjustment up to 15 mm and S3 form for greater adjustment. Spring bars and screw spikes are produced from 60Si2MnA and 20MnTiB, insulators and pads from reinforced nylon 66 and EVA or rubber, and tie plates from QT450-10. High clamping force with good elasticity and insulation is what allows the system to resist the forces generated at high speed.
E-Type Fastening Systems on Plain Line and Mountain Routes
The E-type system is based on the E-type spring clip and is completed by insulating gauge blocks, rubber pads and embedded iron seats. The elastic clip supplies strong clamping pressure, the gauge block maintains gauge and insulates, the pad cushions shock and reduces wear, and the embedded seat supports the assembly. E-clips are made from 60Si2MnA or 60Si2CrA with diameters of 16 mm, 18 mm or 20 mm, hardness of 44-48 HRC and toe load of 500-700 kgf for the 16 mm clip, 750-900 kgf for 18 mm and 1100-1400 kgf for 20 mm, with a fatigue life of five million cycles without fracture. Standards referenced for the clip include DIN 17221, BS 970 and GB/T 1222, while shoulders are cast from QT500-7 or QT450-10 to suit UIC 54, UIC 60, 50 kg and 60 kg rail. The system is used on ordinary trunk lines and, by adjusting parameters, on mountain routes where rail restraint must be increased.
Choosing, Installing and Maintaining the Right System
Match the system to the track form first: ballasted, ballastless, timber sleeper or concrete sleeper each narrows the choice of shoulder and fastener detail.
Check the toe load and pad stiffness against the required track stiffness and the permitted gauge tolerance, rather than selecting on spring bar shape alone.
Confirm thread grade and coating of spikes and bolts against the corrosivity of the environment, and keep insulating components intact because a damaged insulator removes both insulation and gauge control.
Inspect pads and insulators on a fixed interval, since these low-cost items govern how long the clip and the rail remain properly supported.
Frequently Asked Questions
Q: Which fastener system is used on high-speed ballastless track?
The WJ-8 shoulder system is designed for high-speed ballastless track, with high clamping force and W1 or X2 spring bars, rubber or composite pads and embedded sleeves.
Q: What is the difference between a spring bar system and a clamp system?
A spring bar presses on the rail foot through its own elasticity, while a clamp grips the rail foot directly. Both hold gauge, but clamp systems are easier to dismantle for maintenance and line adjustment.
Q: How long should an E-clip last?
E-clips are fatigue tested to five million cycles without fracture, with hardness of 44-48 HRC and toe load between 500-700 kgf and 1100-1400 kgf depending on clip diameter.
Q: Why is electrical insulation part of a fastener system?
Insulating gauge blocks and dowels separate the rail from the sleeper, preventing stray traction current from passing through fasteners and causing electrochemical corrosion or signal interference.
Q: What material are SKL clips made from?
SKL tension clips are made from spring steels such as 60Si2CrA and 38Si7, hardened to about 42-47 HRC, with reference to DIN 17221 and GB/T 1222.
Q: How often do rail pads need replacing?
There is no single interval. Pads are inspected periodically and replaced when they harden, crack or lose resilience, since pad condition determines how effectively the pad absorbs impact and protects the sleeper.