How Railway Rails Are Joined — The Four Methods Explained
- 4 days ago
- 7 min read
When a train passes over track, the rail has to do two things at the same time. It transfers vertical load down into the sleeper and ballast and also forces along its length — from temperature change, traction and braking.
Those forces do not stop at the end of a rail. They must pass through whatever joins one rail length to the next.
Every time two rails are joined, it creates an area where the behaviour of the track changes.
These joins affect:
How forces are transferred
How temperature change and thermal forces are managed
What issues and defects are likely to develop over time
What you should be looking for during inspection
Modern rails can be manufactured in long lengths. But however long they are delivered, they still need to be connected together somehow.
Broadly speaking, there are two main ways rails are joined together:
Mechanical joints
Welded joints
Within these sit four common forms you’ll encounter:
Fishplated joints
Tight joints
Aluminothermic welds
Flash-butt welds
Understanding how each one works helps you recognise:
How it is designed to manage thermal forces
Where it is typically used
What its strengths and weaknesses are
What common defects or issues are associated with it
Mechanical Connections
Fishplated (Jointed) Track
From the earliest development of the railways, rails were installed in shorter lengths and connected together using fishplates and bolts — and this method is still used today, although not as widely as it once was.
Two steel fishplates sit either side of the rail, contacting it top and bottom. These plates are clamped in place with bolts. A gap is left between the rail ends.
That gap is intentional.
The gap size is set based on the rail temperature at the time of installation because it must accommodate the thermal expansion and contraction of the rail. This is how jointed track manages thermal forces and prevents track buckles, with each joint gap allowing every rail to expand and contract as the temperature changes.
To allow this movement, the fishing surfaces of the fishplate are lubricated so the joint can move.
Today, fishplated joints are most commonly found on:
Lower-speed routes
Sidings
Legacy lines not yet upgraded to welded rail
One area where they may still be installed as new, rather than removed and upgraded to welds, is on very tight radius curves. Installing and stressing Continuous Welded Rail (CWR) on tight curves can be difficult due to the forces involved and the challenge of holding the rail in the correct position. In such locations, jointed track is often required by standards and remains a practical solution.
Because the joint gap controls how thermal movement is accommodated, the joints must be monitored.
If a gap is too small, thermal forces can build in hot weather and increase buckle risk.
If a gap is too large, the joint becomes weaker under traffic. In cold weather, as the rail contracts and tension increases, excessive gap can increase stress in the fishplates and raise the likelihood of cracking.
While the gap between rail ends is crucial to how jointed track manages thermal forces, it does introduce a gap in the running surface of the rail — the surface the wheel runs on.
That gap is a weak point in the rail, one that concentrates the load as wheels pass from one rail end to the next.
Over time, this can lead to:
Dipped joints
Battered rail ends
Loosening bolts
Cracked fishplates
Local breakdown of ballast support
That breakdown of ballast beneath the sleeper at the joint is commonly known as voiding. As the track support reduces, the sleeper deflects more under load, which increases stress at the joint and accelerates the fatigue of the fishplates.
Managing joint condition is therefore an ongoing maintenance requirement, not a one-time installation decision.
Tight Joints
Not all mechanical joints are designed to provide an expansion gap.
A tight joint is a fishplated joint where the rail ends are pulled tight together, with no gap between them.
Because the ends are tight up against each other, the joint can be used within stressed track arrangements. A tight joint allows longitudinal forces, such as the tension CWR is put under, to pass through the joint.
To achieve that behaviour, tight joint (TJ) fishplates are used rather than standard plates. They are designed to prevent movement and withstand the forces present in CWR.
You’ll most often see tight joints used when welding cannot be carried out immediately.
Typically this is due to:
Limited possession time
Rail head wear that makes welding impossible or at high risk of failure
Defects in the rail such as rolling contact fatigue
Temporary repairs while waiting for a permanent solution
Even though the rail ends are tight together, it is still a joint and joints still need attention.
Common issues to watch for include:
Local dipping developing at the joint
Rail-end batter (often less pronounced than a gapped joint)
Bolts loosening
Fishplate cracking, or in worst cases complete plate failure
In many cases, tight joints are intended as a temporary measure before replacement with a weld. They are often subjected to limits on how long they can remain in track and require careful inspection while they are present.
Welded Connections – Continuous Welded Rail (CWR)
Mechanical joints were the original solution.
But they introduce gaps in the running surface, bolt holes through the rail web and fishplates clamping the connection together. Each of those elements creates risk that can lead to failure.
Welding removes that joint.
The gap in the rail disappears. The bolt holes disappear. The plates disappear.
Instead, a fused rail section with a continuous running surface is created.
That removes many of the limitations and issues associated with mechanical joints.
But welding introduces its own engineering challenges.
It requires:
Achieving sufficient strength and fatigue resistance to match the rail around it
Producing a consistent microstructure in the new weld material
Maintaining correct alignment of the rails through the joint
Delivering a repeatable process with a reliable outcome
Doing all of the above within the practical constraints of a railway worksite
There are two primary welding processes used on the railway:
Aluminothermic welding (ATW)
Flash-butt welding
Both processes produce a continuous rail, but they get there in different ways.
Aluminothermic Welding (ATW)
Aluminothermic welding, also known as thermite or ATW welding, uses a controlled chemical reaction to produce molten steel.
It utilises the thermite reaction. The reaction involves aluminium powder and iron oxide. When ignited, the aluminium reacts with the oxygen in the iron oxide, producing molten iron and aluminium oxide slag. The reaction generates extremely high temperatures — sufficient to create molten steel that can fuse the rail ends together.
In practice the process involves:
Aligning rail ends
Sealing a mould around the joint
Preheating the rail ends
Igniting the thermite reaction
Allowing molten metal to flow into the mould and fuse the rail ends
After cooling, trimming excess material and grinding the weld to final profile
It is a flexible, site-friendly process.
Because the equipment is portable, thermite welding can be carried out in short possessions, in restricted access locations and without large plant on track.
However, it is a multi-stage process where rail alignment, mould sealing, preheat temperature and final grinding all influence the outcome.
Potential issues can include:
Porosity
Inclusions
Surface profile inaccuracies
Flash-Butt Welding
Flash-butt welding joins the rail ends using electrical resistance and controlled forging pressure.
The rail ends are positioned slightly apart and an electrical current is applied. Resistance at the interface creates an arc and heats the rail ends to a plastic state suitable for forging.
Once the correct temperature is reached, the rails are forced together under pressure. Excess material is expelled and trimmed away, forming a forged joint.
The process is highly mechanised, with alignment, heating, pressure and finishing controlled by the machine.
Flash-butt welding is commonly used:
In rail production plants to create long welded rail (LWR) strings
In switch and crossing fabrication workshops
In rail depots preparing rail for delivery to site
It can also be carried out on track using RRV-mounted flash-butt welding units when access and logistics allow.
Compared with thermite welding, flash-butt welding typically offers:
Faster weld cycle times
Greater repeatability
Reduced finishing work
Excellent suitability for workshop environments
However, the equipment requirements often determine whether it is practical for a given job.
Bringing It Together
Over most of the railway, welded rail is the best overall solution.
Removing gaps and fishplated interfaces reduces built-in weak points and removes a significant source of recurring joint maintenance.
But joints still have a place on the railway.
They remain necessary where constraints on access, time, logistics, geometry or rail condition make welding impractical.
The point is not to label one method good and another bad. Each has its place depending on the circumstances.
Further Learning
I have a number of videos over on my YouTube channel that you should check out.
Why Joint Gaps Matter on Railways — And What Happens If You Get Them Wrong Behind the Spark: The Two Rail Welding Methods That Keep Trains Moving
For all my videos related to rails and rail joints, there is my playlist All Things Rails
Deepen Your Track Knowledge
If you want to properly understand the components that make up the railway track — not just what they look like, but what they do and why they matter — the Track Components ID Guide breaks the system down clearly and simply.
It’s designed to strengthen your knowledge of the track so you’re not just seeing parts — you’re understanding the job each component is doing.
Build a Strong Foundation in Rail
This article focuses on just one element of track engineering.
If you want a structured way to build your understanding of how the whole railway fits together, rather than picking it up in fragments from diagrams and conversations, Intro to Rail provides that foundation.
It walks through the railway in a structured way so you can build knowledge you can apply in day-to-day work.
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