What Is the Primary Function of a Fish Plate in Rail Joints?

What a Fish Plate Does at a Rail Joint

A fish plate, also called a rail joint bar or angle bar, is the heavy steel plate that bolts two rail ends together so that the track behaves as one continuous beam. Its primary function is to join two rail ends while maintaining alignment and continuity of the running line, transferring wheel loads from one rail section to the next and providing the vertical and lateral stiffness that keeps the joint level and prevents the rails from spreading apart under traffic. Every other duty a joint bar performs, from holding gauge to damping impact, follows from that first requirement.

Because the joint is the weakest point of any track structure, the joint bar determines how much impact energy reaches the sleepers, ballast and rail ends. A correctly matched bar spreads that energy over a longer length of rail, which is why joint geometry and bolt tightening are treated as safety-critical in track maintenance manuals.

Core Functions of a Rail Joint Bar

Connection and alignment - clamps adjacent rail ends in the correct horizontal and vertical position so that the running table and the gauge face match across the joint.

Load transfer - spreads the impact and weight of passing wheels over both rail ends, preventing stress concentration in a single section.

Structural integrity - adds stiffness to the joint, resists impact and prevents rail separation or spreading.

Gauge holding - lateral support keeps the two rails at the correct distance apart, which is essential for wheel guidance.

Smooth transition - reduces shock and vibration at the joint, which lowers noise and slows wear in the rail ends and sleepers.

Types of Fish Plates

Joint bars are selected to match the rail section and the electrical requirements of the line.

Standard joint bar - the general-purpose plate that connects two rails of the same section.

Compromise joint bar - joins rails of different sizes or profiles, for example where a 50 kg/m rail meets a 60 kg/m rail, and is essential at transition sections of a track.

Insulated joint bar - separates the two rail ends electrically so that track circuits and signalling systems are not short-circuited through the joint.

Glued insulated joint bar - an insulated bar whose joint faces are bonded so the assembly resists both mechanical and electrical breakdown in high-traffic locations.

Heavy-duty joint bar - a heavier section intended for heavy-haul lines and high axle loads.

Rail Joint Bar Dimensions

Joint bars are quoted by rail weight and by hole pattern. Typical bars for light rail are listed first, followed by heavy rail sections and the AREMA hole patterns used on North American track.

Light rail, 9 to 30 kg/m

Rail size (kg/m)	Length (mm)	Width (mm)	Hole diameter (mm)	Hole count	Weight (kg/pc)
9	385	43.13	14	4	0.81
12	409	46.5	14	4	1.39
15	409	53.54	18	4	2.2
22	510	61.73	22	4	3.8
30	561	71.48	22	4	5.54

Heavy rail, 38 and 50 kg/m

Rail size (kg/m)	Length (mm)	Width (mm)	Hole diameter (mm)	Hole count	Weight (kg/pc)
38 and 43	790	94.03	40	6	15.57
50	820	104.22	46	6	18.72

AREMA hole patterns

Joint bars for AREMA rail sections are usually quoted by hole pattern:

Rail section	Bolt pattern	Hole spacing	Weight
115RE	4 holes	24 in	17.04 kg
115RE	6 holes	36 in	25.56 kg
132RE / 136RE	4 holes	24 in	17.75 kg
132RE / 136RE	6 holes	36 in	26.63 kg

Fitting and Maintenance Practice

Installation is methodical rather than complicated, but the tolerances matter. Rail ends are first aligned and cleaned so that rust, scale or debris cannot sit between the contact faces and the bar. The bar is then seated on the rail web with its holes aligned to the pre-drilled holes in the rail ends, and the bolts are inserted and tightened to the specified torque so that the assembly grips evenly along its whole length.

Check that the bolts are properly fastened and that the bar has no cracks, gouges or bright wear marks on the contact faces.

Keep the bar and the insulation components free from cracks; there should be no grinding, corrosion or bolt-hole ovality beyond the permitted limit.

Confirm the electrical insulation of insulated joints is intact where track circuits are used.

Verify that the joint sits flat with no gap that would allow ballast or debris to enter.

Record repeated joint failures so that sleepers, bolt tension or rail end batter can be corrected before the joint fails again.

Frequently Asked Questions

Q: What is the primary function of a fish plate?
It joins two rail ends and holds them in line, transferring wheel loads from one rail to the next so the track stays continuous and stable.

Q: Can one joint bar connect rails of different sizes?
No. A compromise joint bar is needed when two rails have different sections or profiles, because the contact faces must match both rail webs.

Q: Why are insulated joint bars used?
They stop electrical current from passing between the joined rail ends, which is necessary for track circuits and signalling control.

Q: How are fish plates installed?
The rail ends are cleaned and aligned, the bar is seated on the web, and the bolts are tightened to the specified torque; joints are then inspected at regular intervals.

Q: What sizes are available for light rail?
Light rail joint bars cover rail weights from about 9 to 30 kg/m, with lengths of roughly 385 to 561 mm and unit weights of about 0.81 to 5.54 kg.

Q: What is checked during joint inspection?
Bolt tightness, cracks or corrosion on the bar, ovality of the bolt holes, insulation integrity on insulated joints, and any sign of rail end batter or gauge widening.