An insulated fish plate, also known as an insulated rail joint (IRJ), is a rail joint bar that connects two rail ends while electrically isolating them from each other. It carries the same mechanical duty as a standard fishplate, but adds the signalling function of separating track circuits. Because a failed IRJ can stop both traffic and signalling, its specification must be precise: materials, electrical resistance, mechanical capacity, rail compatibility and configuration all have defined requirements. This article sets out those requirements in the way a purchaser or maintenance engineer would apply them.
1. Construction and Components
An IRJ assembly consists of four functional parts: the insulated fish plates (left and right bars), the insulating end-posts fitted between the rail ends, and insulating bushes and washers on every bolt. The bars are made from high-strength steel, or from bonded composite and epoxy constructions where the insulating layer is part of the bar itself. In glued joints, the bars are bonded to the rail web with high-strength adhesive, which distributes the clamping load evenly and keeps the joint stiff and watertight. Standard bolted joints rely on bolt clamping alone and are used where speed and tonnage are lower.
2. Electrical Specifications
Insulation resistance, dry: minimum 30 megohm for standard joints; glued joints for mainline and high-speed use are specified at 100 megohm (10 to the 8th ohm) or higher when new.
The joint must hold the specified resistance across temperature cycles, rain and ballast contamination; this is why drainage and sealing of the joint area matter as much as the joint itself.
End-posts, bushes and washers are the insulation elements: they must be specified for the rail section and for the track circuit voltage class of the line.
3. Mechanical Requirements
The mechanical duty of an IRJ is demonstrated by type testing rather than by a single figure. EN 16843 covers the requirements for insulated rail joints, including vertical bending performance and repeated-load (fatigue) verification, so a purchaser should require the joint to be type-tested to EN 16843 for the target rail section. In addition, the bars and bolts must be matched to the rail section: for example, a 60 kg/m joint with four bolts per side at 630 mm bar length uses bolt sizes in the 25-28 mm diameter class, and the bolted assembly must develop the clamping force needed to prevent rail movement under braking and thermal loads. The glued variant adds the adhesive bond, which must remain intact under longitudinal rail forces in continuously welded rail.
4. Rail Compatibility and Configurations
| Parameter | Typical values |
| Rail sections | 50 kg/m, 52 kg/m, 60 kg/m; UIC54, UIC60; 115RE, 132RE, 90/91LB; AS 50 kg/m; S49 |
| Configurations | 4-hole assembly, about 610 mm (24 in) long; 6-hole assembly, about 915 mm (36 in) long |
| Bolt pattern | Bolt diameter and spacing follow the rail standard and the fishplate drilling pattern |
| Materials | High-strength steel bars; nylon, fibreglass or epoxy insulating elements |
| Service temperature | Specified for the climate of the line; cold-region joints use low-temperature-grade insulation |
Compromise joints connect two different rail sections at a transition, with each side machined to its own rail profile; these are specified separately from standard IRJs.
5. Specification Checklist for Purchasers
Rail section and standard (GB/T 2585, UIC 864, EN 13674-1, AREMA) to define the bar profile and hole pattern.
Joint type: standard bolted, glued, or compromise joint.
Electrical requirement: minimum insulation resistance in megohm, and whether the value applies dry or wet.
Mechanical requirement: type test to EN 16843 for the rail section, including bending and fatigue verification.
Bolt specification: grade, size, torque and whether insulating bushes and washers are included.
Service temperature range and climate (rain, snow, coastal salt) for material selection.
FAQ
Q1: What insulation resistance should an IRJ provide?
Standard bolted joints are specified at 30 megohm minimum dry; glued joints for mainline use are typically specified at 100 megohm (10 to the 8th ohm) or higher. Confirm the value against the track circuit design of the line.
Q2: Why is a glued insulated joint preferred on mainlines?
The adhesive bond makes the joint stiffer and keeps the insulation layer sealed against moisture, giving stable electrical values and longer mechanical life under heavy traffic and high speeds.
Q3: Which standard governs insulated rail joints?
EN 16843 is the reference for the requirements of insulated rail joints and fish plates, covering the mechanical and electrical verification. Rail sections themselves follow the national or international rail standards.
Q4: Can an IRJ be supplied for two different rail sections?
Yes, as a compromise joint. Each side is machined to its own rail profile so that the transition keeps both alignment and electrical isolation, for example between UIC54 and UIC60.
Q5: What maintenance checks keep an IRJ in service?
Measure insulation resistance at the maintenance interval, check bolt torque, inspect the insulating end-posts, bushes and washers for damage, and monitor rail-end batter and gap growth. Renew the joint when resistance drops below the requirement or the joint shows structural damage.
Q6: Do IRJ bolts need special washers?
Yes. Insulating bushes and washers are part of the assembly and must be installed on every bolt; using plain washers creates a conductive path that shorts the joint and defeats the track circuit isolation.