What the KPO Fastener System Includes
The KPO fastener system is a baseplate-and-clip family that holds the rail foot to the sleeper on ballasted and slab track. A complete assembly combines a gusset-type baseplate, a spring clip, a threaded bolt or wood screw, a plastic insulator and, where the design requires them, a rail pad and a gauge-adjustment shim. Each item carries its own specification, and the specification that suits a given line is set by the rail section, the sleeper type, the design axle load and the design speed rather than by a single universal figure.
KPO3, KPO6 and KPO9 are the three plate sizes most often quoted. The number rises with the bearing area of the plate and with the clip size it accepts, so a larger plate is paired with a heavier rail section and a heavier duty cycle.
KPO3, KPO6 and KPO9 Baseplates
The gusset-type baseplates are normally produced in Q235 carbon structural steel and dimensioned to DIN 5906. The table below summarises how the three sizes are applied in practice.
| Plate | Material | Standard basis | Typical service |
|---|---|---|---|
| KPO3 | Q235 | DIN 5906 | Lines with conventional demands on gauge retention. The plate works with the clip to hold the rail on the sleeper and to resist vertical and lateral movement under passing trains. |
| KPO6 | Q235 | DIN 5906 | Busier main-line track, where a larger plate footprint spreads the clamping force across the sleeper shoulder. |
| KPO9 | Q235 | DIN 5906 | Heavier sections and higher axle loads, where the extra bearing area keeps contact pressure within the capacity of the sleeper. |
All three plates work on the same principle. The rail foot is held between the plate face and the clip, and the bolted connection preloads the assembly so that the clip delivers a continuous toe load. Gauge is maintained because the plate geometry fixes the rail foot laterally, while resistance to longitudinal creep comes from the friction generated by the clamping force.
Bolts, Screws and Insulating Components
Threaded hardware is specified separately from the plate, and the property class of each item governs the preload the fastening can carry.
Wood screws in M24 x 150 mm, M24 x 160 mm and M24 x 180 mm lengths, made in property class 5.6 from 35-grade carbon steel. Tensile strength is at least 500 MPa and yield strength at least 300 MPa, with elongation of at least 20 percent and no cracking after a 90-degree cold bend.
HS bolts in HS26 and HS32 patterns, property class 4.6, Q235 steel, supplied in lengths such as 55 mm, 65 mm, 75 mm and 90 mm so that different installation clearances and force paths can be accommodated.
Insulators moulded in glass-fibre reinforced nylon 66 with a density of 1.30 to 1.45 g/cm3 and tensile strength of at least 170 MPa, providing the electrical isolation required where the fastening must not bridge the track circuit.
Rail sections fitted including UIC 54, UIC 60, 50 kg/m and 60 kg/m profiles, which is what allows one fastening family to cover a broad span of standard track.
The screw material is a medium-carbon steel of the 35 grade, a choice that balances formability during thread rolling against the strength needed for clamping duty. The class 5.6 and 4.6 designations follow the property-class system for metric steel fasteners, in which the first digit gives one tenth of the nominal tensile strength in MPa and the second digit gives the ratio of yield stress to tensile strength.
Matching Specification to Line Conditions
Selection follows a short sequence. First fix the rail section and its foot geometry, because the plate has to accept the foot without rock. Second fix the design axle load and annual tonnage, because these set the toe load and therefore the clip and the bolt class. Third fix the sleeper type, since timber sleepers take wood screws while concrete sleepers take bolted shoulders with cast-in or drilled fixings. Fourth fix whether the line is track-circuited, which decides whether an insulating component is needed at every fastening.
With those four inputs settled, the plate size follows directly. Ordinary lines with moderate demands take KPO3. Main lines with heavier traffic and heavier sections take KPO6. Heavy-haul and high-speed applications take KPO9 together with the correspondingly larger clip and a fully specified hardware set.
Installation and Inspection Points
Check rail foot to plate contact for even seating; local gaps show that the plate or clip does not match the section.
Confirm that the insulator sits correctly, so that the circuit path is broken without the insulator being crushed.
Apply the specified tightening torque. Over-tightening a class 4.6 bolt drives it toward yield and removes the elastic reserve that maintains toe load.
Look for clip set and for fretting marks around the plate holes, which point to movement under repeated loading.
Measure gauge at the fastenings after the first traffic pass, because this is where any mismatch between plate, clip and sleeper shows up first.
Frequently Asked Questions
Q: What is the difference between KPO3, KPO6 and KPO9?
The three sizes differ in bearing area and in the clip they accept. KPO3 suits lines with conventional demands on gauge retention, KPO6 suits busier main-line track, and KPO9 is used with heavier rail sections and higher axle loads.
Q: Which steel is used for KPO baseplates?
The gusset-type plates are commonly made in Q235 carbon structural steel and dimensioned to DIN 5906. Heavier duties are handled by moving up the plate size rather than by changing the grade alone.
Q: What are the standard wood screw sizes in this system?
M24 x 150 mm, M24 x 160 mm and M24 x 180 mm are the usual lengths, in property class 5.6 from 35-grade carbon steel, with at least 500 MPa tensile strength, at least 300 MPa yield strength and at least 20 percent elongation.
Q: Why is a PA66 insulator used?
Glass-fibre reinforced nylon 66 gives high mechanical strength together with electrical insulation. With a density of 1.30 to 1.45 g/cm3 and tensile strength of at least 170 MPa, it isolates the rail from the sleeper in track-circuited territory.
Q: Which rail sections do these fastenings fit?
The system covers UIC 54, UIC 60, 50 kg/m and 60 kg/m rail sections, so a single plate and clip family can serve a wide range of standard track.
Q: How do I choose the right plate size?
Start from the rail section, then the design axle load, the sleeper type and the annual tonnage. Those four inputs decide the toe load and the plate size, so the choice follows a calculation rather than a preference.