What Is a Compromised Fishplate
A compromised fishplate, also known as a combination fishplate, is a joint bar that connects two different rail sections at one joint. When two rail profiles meet, a standard fishplate made for one profile cannot clamp the other, so the joint would be left with a gap, a step or a loose fit. The combination fishplate is machined with two different profiles on the two sides of its centre line: one half matches the heavier rail section, the other half matches the lighter one, and the change happens at the centre point of the plate. The plate is made in varying depths so that the running surfaces of the two rails are brought to the same level, providing a uniform top surface across the joint.
Why Combination Fishplates Are Used
Rail renewal programmes: new heavier rail is laid up to the boundary of existing lighter rail, and the transition joint must connect the two during staged replacement.
Line and siding boundaries: a main line with heavy rail meets an industrial siding or yard laid with lighter rail.
Temporary connections: bridging a short section of different rail while work proceeds, keeping the line open under traffic.
Weight transition: one rail section is lighter and the other heavier, so the joint must absorb the change in sectional area without a hard step.
By eliminating the expansion gap at the transition, the combination fishplate removes the impact point that a conventional gapped joint would create, which reduces damage to both the joint and the rolling stock.
Design and Marking
| Feature | Detail |
|---|---|
| Profile coverage | Half of the plate matches each rail section, transition at the centre point |
| Expansion gap | Eliminated at the transition for a continuous joint |
| Typical combinations | 52 kg / 90R, 90R / 75R, 75R / 60R, 60R / 50R |
| Marking | Right in / right out, left in / left out, per position of lighter and heavier rails |
| Running surface | Uniform top level across the joined rails |
The right in, right out, left in and left out marking system tells the installer which side of the joint each plate belongs to. Because the two rails have different weights and sectional areas, the plates are handed: installing a plate on the wrong side leaves the heavier rail only partially covered and the joint cannot develop its full strength.
Application Scenarios
Combination fishplates are most common in yard and station approaches, where rail sections change between the running line and sidings; in renewal projects, where new rail is joined to old rail still in service; and on industrial tracks where different rail weights meet at plant boundaries. They are also used at temporary crossings where a light rail is bridged by a heavier section under works traffic. In every case the joint is a controlled weak point of the line, so the plate material and bolt quality should be at least equal to the lower of the two rail classes.
Selection and Installation Points
Specify the two rail sections precisely, including the section number and the rail profile drawing, because plates are made to match both profiles. Confirm the handedness when ordering: state whether the lighter rail is on the right or the left when facing the direction of travel. During installation, clean the rail ends and the plate contact faces, align the bolt holes of both rails with the plate, insert the positioning bolts, and tighten in a diagonal sequence to the specified torque so the plate clamps both rails evenly. After installation, check the running surface step: the tops of the two rails must be level within the tolerance in the specification, and the bolts must be re-torqued after the initial traffic period.
FAQ
What does compromised mean in a fishplate name?
It means the plate compromises between two rail profiles: each half matches one section, so the two different rails can be joined in one bolted joint.
Can a combination fishplate be used with rails of very different heights?
Yes, the plate depth varies across its length to bring the running surfaces to one level, provided the sections are within the range the plate is manufactured for.
Why is there no expansion gap in a combination fishplate?
The transition joint is designed as a rigid connection, so the thermal movement of the line is managed elsewhere, in the expansion joints and rail gaps of the surrounding track.
How do I know which way round to install the plates?
Use the right in, right out, left in, left out marking: it refers to the position of the lighter and heavier rails when facing the direction of traffic, as shown in the installation drawing.
Are combination fishplates available for metric and imperial rail mixes?
Yes, combinations such as 52 kg to 90R and 90R to 75R are standard, and mixed metric-imperial pairs can be manufactured to the two profile drawings.