Why the Plate Must Match the Rail Strength
The fishplate and the rail web share the load at a bolted joint: the rail carries the compressive and bending load, and the plate carries the tensile and shear reactions through the bolts. If the plate is weaker than the rail, the plate deforms or fractures first under the passing train, the joint opens and the track loses its continuity. If the plate is much stronger than the rail, the rail end becomes the weak link and the rail web or head suffers damage at the joint under impact. A matched plate deforms together with the rail, spreads the load over the bolt group and keeps the joint stable. That is why both national and international rail systems define the fishplate material together with the rail class, and why a UIC60 rail, for example, is specified with alloy steel fishplates.
Material Classes and Application Scenarios
| Material | Strength level | Typical service |
|---|---|---|
| Q235 carbon steel | Low | Yards, special lines, light traffic and slow speed |
| 45 carbon steel (C45) | Medium | Ordinary-speed main lines, medium axle loads |
| Alloy steel (Cr-Mn alloyed) | High, fatigue resistant | High-speed, heavy-haul, UIC60 and AREMA 136RE rails, curves and turnouts |
Q235 plates are the economical choice where joint loads are small. C45 plates are the mainstream solution for ordinary main lines, balancing cost and strength. Alloy steel plates are required where the joint sees high-speed dynamic forces or heavy axle loads, because their fatigue limit, not just their static strength, is what keeps the joint alive for millions of cycles.
Bolt Hole Design and Joint Integrity
The bolt holes are the weak points of every fishplate, because they remove section area exactly where the stress flows. The number and diameter of holes must follow the rail model: too large a hole or too few holes weakens the effective section and reduces shear capacity. The hole spacing must be uniform so every bolt shares the load equally; uneven spacing overloads one bolt and starts a crack. The holes must line up precisely with the rail web holes, because misalignment leaves a gap between plate and rail, and the joint vibrates and hammers under traffic. The hole edges must be chamfered to remove the stress concentration where fatigue cracks start under long-term vibration. A correctly drilled and chamfered hole pattern is what lets the plate develop its full design strength.
Heavy-Haul Fishplate Requirements
Heavy-haul joints carry repeated impact and large longitudinal forces from heavy axle trains, so three indicators are strengthened. Tensile strength: at least 800 MPa, so the plate resists the longitudinal pulling force at the joint. Shear strength: at least 450 MPa, so the section between bolt holes and the rail web does not shear through. Fatigue resistance: the material structure is optimized by heat treatment so the plate survives millions of load cycles without crack initiation. Surface hardness of HRC 30-35 improves wear resistance at the plate-rail contact, and the periphery of the bolt holes is thickened to carry the concentrated bearing load. These values are the design basis for heavy-haul fishplates and should be confirmed in the material specification of any order for such lines.
Installation Procedure
Clean the rail ends and the plate contact faces of rust, oil and debris before assembly. Align the plate holes with the rail web holes, insert two or three positioning bolts, then adjust the plate with a crowbar so it seats fully against the rail; the maximum allowed gap is 0.2 mm. Tighten the bolts in a diagonal cross sequence in three passes so the preload builds evenly and the plate does not warp. On old rails, grind the rail ends first if they are worn or deformed, so the plate seats on a flat surface. After installation, re-check the contact with a feeler gauge; if any gap exceeds the standard, readjust or replace the plate.
Strength-match the plate to the rail class; a mismatch in either direction creates a weak point at the joint.
Bolt hole number, diameter, spacing and chamfer decide the effective section of the plate.
Heavy-haul plates need a tensile strength above 800 MPa, shear above 450 MPa and HRC 30-35 hardness.
FAQ
Why is a stronger fishplate not automatically better?
Because joint design is a match: if the plate is much stronger than the rail, the rail end becomes the weak point and takes the damage under impact. The plate should be matched to the rail class.
What is the maximum gap allowed between plate and rail?
0.2 mm after final tightening. Larger gaps mean the plate is not carrying the load and the joint hammers, which damages the rail end and the bolts.
Can Q235 fishplates be used on a main line?
Only on very light secondary traffic; for main-line service use at least 45 carbon steel, and alloy steel for high-speed or heavy-haul lines.
Why are hole edges chamfered?
Chamfering removes the sharp corner where stress concentrates, preventing the edge cracks that start under repeated bending of the joint.
How is the bolt torque sequence performed?
In a diagonal cross sequence in three tightening passes, so the preload rises evenly and the plate is pulled flat against the rail instead of warping from one side.