Fishplate Specifications

The integrity of any railway track depends on the strength and reliability of its connections. While modern techniques like thermite welding create seamless tracks, the traditional bolted joint remains a fundamental component in many rail networks. At the heart of this connection is the fishplate, also known as a joint bar or splice bar. This seemingly simple steel plate is a critical engineering component responsible for securely joining the ends of two rails, ensuring track continuity, alignment, and safety. Understanding the specifications, types, and installation of the fishplate is essential for anyone involved in railway construction, maintenance, or engineering.

Fishplate Types and Functions

A fishplate is more than just a steel bar with holes; it is precisely engineered to fit snugly against the web of the rail section, held in place by high-strength track bolts. Its primary function is to transfer loads between the two rail ends, maintaining a smooth and continuous running surface for train wheels. By clamping the rails together, fishplates resist vertical, lateral, and longitudinal forces, preventing misalignment and ensuring the track gauge remains consistent.

There are several types of fishplates, each designed for specific applications and track conditions:

• Standard Fishplates: These are the most common type, consisting of a pair of bars bolted to each side of the rail web. They are designed to match the profile of a specific rail section (e.g., 115 RE, 136 RE) to ensure a perfect fit. They typically have four or six bolt holes.
• Joggled Fishplates: This specialized type is used for temporary repairs or to join rails of slightly different wear levels. A “joggle” or bend is forged into the plate to accommodate the misalignment, ensuring a smooth transition. They are essential for maintenance crews to quickly repair a broken rail and restore service.
• Compromise Fishplates: When two different rail sections need to be joined (for example, connecting a 115 RE rail to a 132 RE rail), a compromise fishplate is required. These are custom-manufactured with a unique profile that transitions from one rail size to the other, ensuring a secure and safe joint.
• Insulated Rail Joints (IRJs): In tracks with signaling systems, it is necessary to electrically isolate sections of the rail. An insulated rail joint uses fishplates made from a non-conductive composite material or standard steel fishplates with insulating end posts and bushings. This prevents the flow of electrical current between rail sections, allowing track circuits to operate correctly.

The design of a fishplate, especially its cross-section, is tapered to wedge tightly into the fishing angle of the rail web and head. This creates an extremely rigid connection that mimics the strength of an unbroken rail.

Fishplate Specifications According to AREMA Standards

The American Railway Engineering and Maintenance-of-Way Association (AREMA) sets the standards for rail components in North America, including fishplates. These specifications ensure that fishplates have the correct chemical composition, mechanical properties, and dimensional accuracy to handle the immense stresses of railway traffic. The dimensions of a fishplate must precisely match the rail section it is intended for. A mismatch can lead to a loose joint, accelerated wear, and potential derailment.

The length of the fishplate and the number of bolt holes depend on the application. A standard four-hole fishplate provides a strong joint for most mainline applications. For heavy-haul lines, high-speed corridors, or areas with extreme temperature fluctuations, a longer, six-hole fishplate is often used. The additional bolts provide greater clamping force and better resistance to longitudinal movement (rail creep).

The table below outlines typical specifications for fishplates designed for common AREMA rail sections. These dimensions are crucial for ensuring compatibility and safety.

The material used for fishplates is typically high-carbon steel that has been heat-treated to achieve a balance of strength and ductility. The steel must be strong enough to resist bending and fracture under load but also ductile enough to withstand the impact and vibration from passing trains without becoming brittle.

Installation and Maintenance of Bolted Joints

Proper installation is fundamental to the performance and longevity of a bolted rail joint. An incorrectly installed fishplate can lead to premature failure of the joint, damage to the rails, and unsafe track conditions.

The installation process involves several key steps:

1. Preparation: The rail ends and the fishplates must be clean and free of any rust, scale, or debris. The fishing surfaces of the rail (the area where the fishplate makes contact) should be lubricated to allow for longitudinal expansion and contraction of the rails with temperature changes.
2. Assembly: The pair of fishplates are placed on either side of the rail web, and the track bolts are inserted through the holes. Washers, such as spring washers, are used to maintain tension and prevent the nuts from loosening under vibration.
3. Bolting Pattern: The bolts are tightened in a specific sequence to ensure even clamping pressure. A common pattern is to start with the inner bolts and work outwards, alternating between the bolts on each rail end.
4. Torquing: The bolts must be tightened to a specific torque value using a calibrated torque wrench. Under-tightening can lead to a loose joint, while over-tightening can stretch the bolts and damage the fishplates or rails. Recommended torque values depend on the bolt size and rail section.

Maintenance of bolted joints is an ongoing and critical task for railway track inspectors and maintenance crews. Joints must be regularly inspected for:

• Loose or missing bolts: Vibration from trains can cause nuts to loosen over time. All bolts must be checked and re-torqued as necessary.
• Cracks or fractures: Fishplates and rail ends are subject to high stress and can develop fatigue cracks. Any signs of cracking require immediate replacement of the component.
• Wear and corrosion: The fishing surfaces of the rail and fishplate can wear down over time, leading to a loose fit. Corrosion can also weaken the components and should be monitored.
• Low joints: A worn or loose joint can cause the rail head to dip, creating a rough ride and high impact loads on the track structure. Shims can sometimes be used to correct minor height differences, but severe wear requires component replacement.

While continuous welded rail (CWR) has become the standard for modern high-speed and heavy-haul lines due to its lower maintenance needs and smoother ride, the fishplate remains an indispensable component for creating temporary joints, connecting special trackwork, and in areas where CWR is not practical. Its robust, simple design has ensured its place in railway engineering for over 170 years.