A fish plate connection, also called a rail joint, is a metal splice-bar joint used in railway engineering to connect two rails end-to-end. Bolted to the sides of the rail web, the steel plates maintain structural continuity, alignment and stability for trains while allowing for thermal expansion and contraction of the track.
How a Fish Plate Connection Works
Typically, two steel plates are placed on opposite sides of the rail joint, one on the inner side and one on the outer side, and secured with high-strength bolts passing through the rail web. The plates are made of rolled or forged carbon steel to withstand heavy loads. Their primary role is to ensure a smooth, secure connection that keeps the rails aligned vertically and horizontally.
Common Types of Fish Plate Connections
Several types of fish plates exist to meet different track requirements. The standard joint bar has a flat profile with a 4-bolt or 6-bolt pattern. The compromise joint bar connects two different rail sections where the profile changes. The insulated joint bar electrically isolates track circuits and is made with composite materials or coated steel. The joggled joint bar has a depressed section that allows smooth wheel passage over the gap.
Typical Specifications.
Fish plate specifications are defined by the rail section they fit. Standard lengths are typically 600 mm, 800 mm or 1000 mm per side; longer bars provide greater stiffness. Bolt holes usually number 4 or 6 per bar side, with common bolt diameters of 22 mm, 24 mm or 1 inch, and standardized hole spacing of about 150 mm. Applicable rail series include UIC 54 and UIC 60, BS 75R to BS 100A, AREMA 115RE and 136RE, GB 50 to 75 kg/m, AS 50 to AS 68, and crane rail QU70 to QU120.
| Parameter | Typical Value |
|---|---|
| Standard length | 600 / 800 / 1000 mm per side |
| Bolt holes | 4 or 6 per bar side |
| Bolt diameter | 22 mm, 24 mm or 1 inch |
| Hole spacing | About 150 mm |
Key Benefits
Fish plate connections distribute wheel loads evenly across the joint to minimize wear, prevent misalignment that could lead to derailments, and allow slight expansion and contraction in varying temperatures. They also permit quick removal of rail sections for repair, which simplifies maintenance on jointed track.
Selection, Installation and Maintenance
Choose the fish plate type according to the rail section, traffic conditions and whether signalling isolation is required. During installation, align the rail ends, insert the bolts with the correct orientation, and tighten to the specified torque in the proper sequence. Leave the correct expansion gap between rail ends according to the rail temperature and the governing rules.
Inspect joints for cracked plates, loose or corroded bolts, and excessive rail end gap. Re-tighten bolts after traffic settlement, protect plates from corrosion, and replace damaged components promptly. A well-maintained fish plate connection has stiffness close to the parent rail and avoids dipping, buckling and wheel jumping at the connection point.
Common Misconceptions
One misconception is that the joint is as strong as a welded connection; in practice, joints are the least stiff point of the track and need more attention. Another is that one fish plate type fits all rails; plates must match the exact rail profile and bolt pattern.
Frequently Asked Questions
What is a fish plate connection? bolted splice-bar joint that connects two rails end-to-end while maintaining alignment and allowing thermal movement.
What are the common types of fish plates? Standard, compromise, insulated and joggled joint bars.
What is an insulated joint bar? fish plate that electrically isolates track circuits, made with composite materials or coated steel.
How long are typical fish plates? Commonly 600 mm, 800 mm or 1000 mm per side, with 4 or 6 bolt holes.
Why leave a gap between rail ends? To allow thermal expansion and contraction without buckling or warping the track.
