How Are Rails Joined? Welding vs Mechanical Joints Explained

Rails are joined in two fundamentally different ways: by welding them together to form continuous welded rail (CWR), or by mechanically fastening them with fish plates (joint bars) and bolts. Modern high-speed and heavy-haul mainlines are almost entirely welded, because a weld gives a seamless running surface with less maintenance and noise. Bolted joints are still used at turnouts, bridges, temporary tracks and mining lines, where thermal movement must be accommodated or where the track may be dismantled.

Welding Methods

Thermite Welding (Aluminothermic)

Thermite welding uses a chemical reaction between iron oxide and aluminium powder to produce superheated molten steel. The molten metal is poured into a mould around the prepared rail ends, fusing them together. Thermite welding is portable and is used on site to close the final gaps in long welded strings and to repair rail. The weld is then ground flush and inspected, and its mechanical properties must be equal to or greater than the parent rail.

Electric Flash-Butt Welding

Flash-butt welding is an automated process in which the rail ends are clamped, heated by electric current until molten, and then pressed together to form the weld. It is used in fixed welding plants and in mobile welding trains, and produces very consistent, high-quality welds. Flash-butt welding is the preferred method for building long welded strings and for joining new rail sections.

Mechanical Joints (Fish Plates / Joint Bars)

Traditionally, rail ends are connected by attaching two steel plates - fishplates or joint bars - on both sides of the rail web and bolting them together with high-strength fish bolts, nuts and spring washers. The plates are machined to match the rail section, so the joint maintains alignment, gauge and load transfer. Bolted joints are designed with a small gap between the rail ends to allow thermal expansion and contraction; the gap is set according to the rail temperature at installation.

Specialty Joints

Insulated joints: used in signaling districts to break electrical conductivity while keeping the track mechanically continuous.

Compromise joints: connect two rail sections of different sizes, with each half of the joint bar machined to its own rail profile.

Joggled joints: a depressed section keeps the rail head surface continuous for smoother wheel passage.

Glued insulated joints: bonded with high-strength adhesive for extra rigidity and durability.

Expansion joints: allow controlled thermal movement in long rails on bridges or in extreme temperature zones.

Joint Type Specification Table

Joint Type	Key Specification / Description
Standard rail joint (fish plate)	Profile matches the rail section (e.g. UIC60, 136RE); length typically 600mm or 800mm per side; 4 or 6 bolt holes per bar
Insulated rail joint (IRJ)	Steel core with composite insulating end posts/plates, or full epoxy composite; electrical resistance above 10^8 ohms
Compromise rail joint	Connects two different rail profiles (e.g. UIC54 to UIC60); each side machined to its respective rail section
Joggled rail joint	Depressed (joggled) section keeps a continuous rail head surface for smoother wheel passage
Bridge rail joint	Reinforced with thicker web or longer length for high-stress locations such as bridge approaches
Glued insulated joint	Steel bars bonded to composite insulation with high-strength adhesive for improved durability
Welded rail joint (CWR)	Flash-butt or thermite weld; weld properties must equal or exceed the parent rail
Expansion joint (BRE type)	Allows controlled thermal expansion in long rails on bridges or in extreme temperature zones

The Function of a Rail Joint

A rail joint connects two adjacent rail ends, ensuring a continuous and stable track for trains. It maintains precise horizontal and vertical alignment, allows thermal expansion and contraction, supports the rail ends against the load of passing wheels, and distributes the pressure across the joint so that no single component is overloaded. In signaling areas, insulated rail joints separate the track into detection blocks, so the joint also has an electrical function.

Welded vs Bolted: Which to Choose?

Welded (CWR): smoothest ride, lowest maintenance, fewer impact loads, preferred for mainlines and high-speed lines; requires continuous welded rail planning, stress-free temperature control and expansion provisions at structures.

Bolted: simple to install and maintain, allows dismantling and thermal movement, standard for light rail, mining, industrial, temporary and yard tracks; needs periodic bolt re-tightening and joint maintenance.

Frequently Asked Questions

1. Why is thermite welding used on site instead of flash-butt welding?

Flash-butt welding needs heavy plant and a long, straight working area, so it is used in plants and welding trains. Thermite welding is portable - the mould, crucible and materials fit in a small trolley - which makes it practical for closing the last gaps in a welded string and for repair work in restricted space.

2. How is the joint gap set in a bolted rail joint?

The gap is calculated from the rail temperature at installation, the rail length and the local temperature range, so that the joint neither closes up in heat nor opens too far in cold. The fish plate and bolts hold the two rail ends at the designed gap.

3. Can welded rail be repaired with a bolted joint?

Yes, temporarily. A joggled fish plate or standard joint bar can bridge a broken or defective weld until the section can be re-welded, keeping the line open at reduced speed under the applicable restrictions.

4. What is the maximum length of continuous welded rail?

In practice CWR is installed in long strings of several hundred metres to a few kilometres, separated by expansion provisions at bridges, tunnels and turnouts. The limiting factor is the thermal force the track structure can withstand without buckling.

5. Do welded joints need maintenance?

Welds are inspected with ultrasonic and surface testing on a scheduled basis, and the rail around welds is monitored for defects, but there is no re-tightening or gap maintenance as with bolted joints, which is the main maintenance advantage of CWR.

6. Which joint type is used at railway turnouts?

Turnouts use a combination: the switch and crossing rails are joined with fish plate joints or welds depending on the rail fastening system, and insulated joints are installed where the turnout connects to track-circuited sections. Compromise joints may be used where the turnout rail section differs from the mainline section.