What Is a Rail Joint? Structure, Function and Types
Jan 28, 2026
A rail joint is a mechanical connection used to link two adjacent rail ends so that they function as a continuous track section and train wheels pass smoothly from one rail to the next. It is installed at the gap between rail ends and is designed to maintain alignment, gauge and load-transfer continuity while accommodating the limited movement caused by temperature changes and operational forces.
How a Rail Joint Is Constructed
Structurally, a rail joint works by clamping the two rail ends together with joint bars (fish plates) on both sides of the rail web, secured by high-strength rail bolts and nuts. The joint bars are machined to match the fishing surfaces of the rail - the underside of the head and the top of the base - so that the running surface and gauge face of the two rails stay level. Spring washers keep the bolts tight as the joint settles under traffic. Although welded rails are now widely used, rail joints remain necessary at turnouts, bridges, temporary tracks, mining railways and wherever rail replacement or thermal movement must be accommodated.
Functions of a Rail Joint
Load transfer: vertical wheel loads and lateral forces are carried across the rail gap so that stress is not concentrated at the rail ends.
Alignment control: the joint prevents the rail ends from shifting independently, maintaining gauge, straightness and level.
Thermal movement: bolted joints allow controlled longitudinal expansion and contraction of the rails with temperature change, reducing the risk of buckling or tensile fracture.
Impact reduction: a well-maintained joint minimizes the step and gap that would otherwise generate impact forces at wheel passage.
Forms of Rail Joint
Traditional bolted joints are common on light rail, mining rail and temporary tracks. For higher-speed or heavier-load lines, compromise joints connect rails of different sections, and insulated joints provide electrical separation for signaling. On modern heavy-haul and high-speed railways most mainline track uses continuous welded rail to eliminate joints over long distances, but joints are still unavoidable at special trackwork and maintenance interfaces.
Typical Compromise and Special Joint Configurations
| No. | Joint Type | Material | Unit Weight (kg/pc) | Remark |
|---|---|---|---|---|
| 1 | 136RE to 115RE | 55# | 27.54 | pcs per set |
| 2 | 136RE to 132RE | 55# | 26.72 | pcs per set |
| 3 | 141RE to 136RE | 55# | 26.72 | pcs per set |
| 4 | 141RE to 132RE | 55# | 26.72 | pcs per set |
| 5 | 100-8, 1/8in vertical offset | 55# | 15.62 | pcs per set |
| 6 | 100-8, 3/8in vertical offset | 55# | 15.62 | pcs per set |
| 7 | UIC60 to P50 | 55# | 23 | pcs per set |
| 8 | BS80A to 115RE | 55# | 20 | pcs per set |
| 9 | UIC60 thermit welded joint | 55# | 18 | pcs per set |
The configurations above cover the common transitions between AREMA (100-8, 115RE, 132RE, 136RE, 141RE), UIC (UIC60, P50) and BS (BS80A) rail sections. Material and bar length can be customized to the buyer's request.
Why Jointed Track Is Still Needed
Even on modern networks, bolted joints are required wherever the track must be demountable, where thermal movement must be absorbed at structures, and where the alignment cannot be welded in a single campaign. Yards, depots, sidings and mining lines are almost universally jointed, and jointed track is faster to install and cheaper to maintain in small sections than CWR.
Frequently Asked Questions
1. What is the difference between a rail joint and a fish plate?
A rail joint is the complete connection, while the fish plate (joint bar) is the steel bar bolted to the rail web. The joint consists of two fish plates, the fish bolts, nuts, washers and the rail ends themselves.
2. Why is a small gap left between rail ends?
The gap allows the rails to expand and contract with temperature. If the ends touched, the rail would buckle in heat; if the gap were too large, wheels would impact the joint. The correct gap is calculated from the installation temperature and the local climate.
3. What is a compromise rail joint used for?
It connects two rails of different sections, such as a 136RE rail to a 115RE rail, or UIC60 to P50. Each half of the bar matches its own rail profile, with a tapered transition on the running surface.
4. How often must bolted rail joints be inspected?
Bolted joints are inspected at every track patrol for bolt tightness, plate condition and gap, and receive a detailed inspection in each maintenance cycle. Joints in high-traffic or high-impact locations need more frequent attention.
5. Can a rail joint be insulated for signaling?
Yes. Insulated rail joints place composite end posts, liners and bolt sleeves between the steel parts so the two rails are electrically separated while staying mechanically connected, with electrical resistance typically above 10^8 ohms.
6. What material is used for joint bars?
Joint bars are normally made from 45# or 55# medium-high carbon steel, heat-treated where required for higher strength. The material must be weldable or machinable to the rail profile and able to carry the joint bending loads.
