What Rail Joints Do
A rail joint connects two rail lengths and must do four jobs at once. First, it transfers the vertical wheel load from one rail end to the next without a step. Second, it allows the rails to expand and contract with temperature - a 100 m length of rail moves about 12 mm for a 50 °C temperature swing, and the joint gap absorbs that movement. Third, it keeps the rail ends aligned in both gauge and vertical planes. Fourth, in signaling areas the joint must break the electrical circuit between adjacent track sections. Every one of these functions depends on the same hardware: two fishplates bolted to the rail web, with the bolt preload doing the work of keeping the assembly rigid.
Types of Rail Joints
| Joint Type | Construction | Where Used |
|---|---|---|
| Standard fishplated joint | Two steel fishplates bolted through the rail web, 4 or 6 bolts per bar | Jointed secondary lines, industrial sidings, temporary track, transition zones |
| Compromise joint | Each end of the plate matches a different rail section | Where two rail profiles meet, e.g. 50 kg/m to 60 kg/m |
| Insulated rail joint, or IRJ, | Fishplates plus an insulating end post and insulating bushings | Track circuit boundaries, signal sections |
| Expansion joint | Sliding rail end with a supported tongue rail | Bridges and long CWR zones in extreme temperature regions |
| Welded joint in continuously welded rail, or CWR, | Flash-butt or thermite weld, no mechanical joint | Continuously welded main lines |
What Happens at a Joint Under Traffic
Every wheel crossing the joint gap strikes the far rail end; the impact force rises with speed and with the size of the gap and any vertical mismatch. The fishplate carries the bending moment across the gap, and the bolts transfer shear between rail web and plate. If bolt preload decays, the fishplate works loose, the joint dips, and the impact grows - a self-reinforcing cycle that batters the rail ends, cracks the fishplates and eventually breaks bolts. This is why joint maintenance is a torque discipline first and a metal inspection second.
Joint Defects and How to Detect Them
Dipped joint: the rail ends settle below the running surface; detected with a 1 m straightedge and feeler gauge, corrected by grinding or joint rebuilding.
Rail end batter: plastic deformation and crushing of the rail end surface from impact; visible as a bright, flattened zone.
Loose or broken bolts: found by torque check and by listening for the characteristic rattle; a single broken bolt doubles the load on its neighbour.
Fishplate cracks: usually start at the bolt holes; found by visual inspection, magnetic particle testing or ultrasonic examination of the plate web.
Corrosion and rust packing: water trapped between plate and rail web; found when the joint is opened during reconditioning.
Maintenance Regime for Jointed Track
Re-tighten joint bolts with a calibrated torque wrench to the design torque for the fishplate size; check torque at least quarterly, and monthly on heavy-haul lines.
Measure the joint gap at the season's extremes - the gap must stay within the designed range for the rail length and temperature zone.
Lubricate the rail web and fishplate faces at the specified interval so water and corrosion products do not freeze the joint together.
Replace worn fishplates in pairs, with bolts and washers renewed at the same time; never reuse bolts from a loosened joint.
Track the vertical profile at joints with a straightedge at every inspection and grind high spots before they become batter.
Jointed Track or CWR?
Continuous welded rail removes most joints and their maintenance, but jointed track remains the right choice where welding is impractical: short sidings, curved industrial track that will be relaid soon, temporary diversions, and sections that need frequent removal. Where CWR is used, expansion joints are installed only at the boundaries - bridges and the ends of long welded zones in extreme temperature regions - because the rail cannot move freely inside the welded length.
Common Misconceptions
"The joint gap is a defect." The gap is a design feature that absorbs thermal expansion; a joint closed to zero gap in summer will push or buckle in the next heatwave.
"Tightening bolts harder is always better." Over-torquing crushes the rail web and cracks fishplates at the bolt holes; the design torque is an upper limit as well as a lower one.
"Insulated joints are only for metro lines." Every track-circuited line, including plain main lines, uses insulated joints to separate signal sections.
"A cracked fishplate can wait until the next maintenance window." A cracked fishplate is a safety-critical defect; the section should be speed-restricted and the plate replaced promptly.
FAQ
Why do rail joints need a gap?
Steel expands with heat. The gap lets each rail length grow without pushing against its neighbour, which would cause the track to buckle; in winter the gap closes as the rail contracts.
How is a dipped joint corrected?
By grinding the high rail end to restore the profile, or by rebuilding the joint - changing pads, fishplates and bolts - when the dip is caused by a loose assembly rather than wear.
What torque should joint bolts be tightened to?
The design torque for the fishplate and bolt size, typically in the range given by the fishplate standard or the design drawing; the value is verified with a calibrated torque wrench during installation and inspections.
Why do fishplates crack at the bolt holes?
The bolt holes are stress concentrators, and any loss of bolt preload lets the plate flex under every wheel; the fatigue cracks then start at the hole edges. Correct torque and periodic re-tightening are the prevention.
Can a compromise joint connect rails of different heights?
A compromise fishplate matches the web profiles and hole patterns of two sections, and a shim under the lower rail foot or a transition pad is used to level the running surfaces. The joint is then treated as a speed-restricted transition point.