The Load Path Through a Rail Joint
At a joint, the rail ends are discontinuous, so the bending moment carried by the rail must be transferred through the fishplates and the bolts. The fishplates act as a pair of beams clamped to the rail web, and the joint stiffness depends on the fishplate section properties, the contact condition between plate and rail, and the preload in the bolts. Under a passing wheel, the joint deflects downward, then rebounds, and this cyclic deflection is what drives fatigue. A light plate with the same outer envelope but a thinner web will have lower second moment of area, so the joint deflects more, impact factor rises, and both the plate and the surrounding track deteriorate faster. Design therefore starts from a required joint deflection or stiffness, not from a weight target.
Section Optimisation Rather than Simple Thinning
Because bending stiffness scales with the second moment of area, material near the extreme fibres contributes most. A lightweight design moves material away from the neutral axis and into the flanges, using a taller, thinner web and shaped fillets, which is why modern high strength fishplates often have a more pronounced top and bottom profile than older rolled sections. Bolt hole regions must remain locally thick because the net section at the holes governs tension and bearing stress. Transition radii between the body and the ends and around the bolt holes control the stress concentration factor, and these radii are the usual location of fatigue initiation. A useful lightweight design reduces mass in the web, where stress is low, and leaves metal where the load is carried.
Materials and Manufacturing Routes
Fishplates are made by rolling, forging or machining from plate. Forged plates allow the section to be tailored along the length and give favourable grain flow, which improves fatigue performance around the bolt holes. Material is typically a medium carbon or low alloy steel with controlled hardenability, quenched and tempered to a specified strength and toughness range; the rail section standards in the EN 13674 series define rail grades, and joint bars are usually specified by their own mechanical property requirements rather than by rail grade. Where a joint must be both light and strong, the design often uses a higher strength steel at reduced section, provided toughness is maintained so the plate does not become notch sensitive. Heat treatment, straightness and hole quality all need tight control because a lightweight plate has less reserve material to tolerate a defect.
Bolts, Preload and Joint Integrity
A high strength joint depends on bolt preload to clamp the plates to the rail web. Preload must be high enough to keep the joint faces in contact under bending so the bolts see a reduced load range, which is the classic reason preloaded joints outlast loose ones. Bolt class, diameter, hole clearance and the use of hardened washers all matter. Where the joint is also a traction return path, the electrical contact between plate and rail must be maintained, which constrains the use of insulating coatings on the faying surfaces. For a lightweight joint, the bolt spacing available is fixed by the standard rail drilling pattern, so the design must work within those constraints; the usual outcome is a plate optimised for the standard pattern rather than a new hole arrangement.
Validation Before Service
Validation follows a staged approach. Section properties and stress under a defined wheel load are calculated, then a full scale static and fatigue test is performed on a joint assembly, applying cyclic bending at a load range representative of service and continuing to a defined number of cycles or to failure. The joint deflection under load is measured, because it is the parameter most closely linked to track performance, and the failure location is recorded to confirm the design assumptions. Field trials on an instrumented joint measure impact factor and compare it with a conventional bar. Only after static, fatigue and field evidence agree should a lightweight design be released, and the release should state the traffic and axle load range it was validated for.
Frequently Asked Questions
Q: What limits how light a fishplate can be?
Bending stiffness and fatigue strength at the bolt holes and section transitions; reducing mass below the value needed to control joint deflection increases impact loading.
Q: Why are flanges preferred over a thick web?
Material at the extreme fibres contributes most to the second moment of area, so a taller section with a thinner web gives more stiffness per unit mass.
Q: How are high strength fishplates made?
By forging or machining from plate in a controlled hardenability steel, quenched and tempered to a defined strength and toughness range.
Q: Why is bolt preload critical in a lightweight joint?
Preload keeps the plate faces in contact so the bolts see a smaller load range; loss of preload exposes the bolts and the plates to full cyclic bending.
Q: How is a new fishplate design validated?
By calculation, then static and fatigue testing of a full assembly, followed by instrumented field trials confirming joint deflection and impact factor.
Q: Which standard covers rail sections?
The EN 13674 series covers railway rail sections and grades, and joint bars are usually specified by their own mechanical property requirements.