Rail bolts are the threaded fasteners that hold the track system together. Their primary role is to clamp fishplates to the rail web at bolted joints, transmitting bending and shear across the joint, but the same fastener family is used for clips, clamps, baseplates, turnouts and rail anchors. This article explains what rail bolts are, the main types, how they are constructed, and how to select the right bolt for a joint or fastening application.
1. What Rail Bolts Do
At a bolted rail joint, the fishplates (joint bars) sit on both sides of the rail web and are clamped by bolts passing through the fishplate and rail web holes. The bolt preload generates friction between fishplate and rail, which is what actually carries the load; the bolt also positions the fishplate against the rail. Away from joints, similar bolts secure clips, clamps, tie plates, turnout rails and anti-creep devices. Because a loosened joint bolt changes the joint stiffness and accelerates rail-end batter, bolt specification and torque control are maintenance-critical.
2. Main Types of Rail Bolts
Button head oval neck bolt: the international standard fishplate bolt. The oval neck seats in the oval hole of the fishplate and stops the bolt from turning while the nut is tightened.
Diamond neck bolt: the square or diamond neck locks into the fishplate slot, used where higher clamping stiffness is required.
Fish bolt: general term for joint bolts supplied with nuts and spring washers, matched to the fishplate hole pattern.
Clip bolt and clamp bolt: secure rail clips and clamp assemblies to the sleeper or baseplate; thread and length follow the clamp design.
T-bolt: dropped into the channel of a baseplate or rail clamp; tightened with a square or hex nut.
Anchor bolt and inserted bolt: fix baseplates, turnout components and embedded rails; inserted types with special threads are used in Chile-standard track.
National standard types: NF F 50-008 covers French track bolts, and GOST-based bolts follow Chile practice; each has defined head, neck and thread geometry.
3. Structure of a Track Bolt Assembly
A track bolt assembly has four elements. The bolt shank carries the external thread at one end and the head at the other; the head shape, button or square, defines how the bolt is held during tightening. The nut, normally hexagonal, engages the thread and applies the clamping force. Washers sit under the nut (and often under the head) to spread the load over the fishplate and protect it from galling. Anti-loosening elements, such as spring washers, lock washers or lock nuts, prevent the nut from backing off under train vibration; for joint bolts, spring washers are the standard choice because they also compensate for the small settlements of the joint.
4. Materials, Grades and Coatings
Bolt steel is selected by property class per ISO 898-1: class 8.8 and 10.9 are typical for track bolts, with 10.9 preferred for heavy-haul and high-speed fastenings where preload is high. Stainless steel bolts per ISO 3506 are used in coastal and corrosive environments. The most common coating is zinc: hot-dip galvanizing per EN ISO 1461 for thick protection, electroplating per ISO 4042 for closer tolerances, or zinc-flake coating per ISO 10683 where hydrogen embrittlement must be excluded. Threads are often left uncoated or lightly lubricated so the friction coefficient, and therefore the achievable preload, is predictable.
5. Selection and Torque Practice
Three parameters define the bolt: diameter, length and property class. Diameter follows the fishplate or fastener hole pattern, typically 22-27 mm for 50-60 kg/m rail joints. Length must cover the fishplate, the rail web and the nut with at least two full threads proud. The property class sets the tightening torque, which is normally defined by the fastening system documentation; on site, torque wrenches are used and loose bolts are re-tightened on the maintenance cycle. Never mix property classes in one joint, and never reuse a bolt that has been over-torqued or shows thread damage, since preload control is what keeps the joint tight under traffic.
FAQ
Q1: Why do rail joint bolts have an oval neck?
The oval neck seats in the oval hole of the fishplate and prevents the bolt from rotating while the nut is tightened. This allows one-person installation and guarantees that the bolt head is correctly oriented in the fishplate slot.
Q2: What is the difference between a rail bolt and a fish bolt?
In everyday use the terms overlap. Fish bolt is the specific term for the bolt that clamps fishplates at a rail joint; rail bolt is the broader family that also includes clip bolts, clamp bolts, T-bolts and anchor bolts.
Q3: Which bolt grade should I specify for heavy-haul joints?
Property class 10.9 per ISO 898-1 is the standard choice for heavy-haul and high-speed joint bolts because the higher yield strength allows a higher preload, which keeps the fishplate friction connection tight under large wheel loads.
Q4: Why do track bolts loosen under traffic?
Cyclic wheel loads cause tiny settlements in the joint, vibration acts on the thread, and temperature changes alter the rail length. Spring washers and lock elements compensate for the settlements, and regular torque checks on the maintenance cycle catch the rest.
Q5: Can I use ordinary structural bolts for rail joints?
Only if the geometry matches. Joint bolts need the oval or diamond neck that locks into the fishplate, plus a defined grip length and a corrosion treatment suited to outdoor track service. A standard hex bolt without the neck will rotate and cannot develop the designed preload.
Q6: How do I know the tightening torque for a track bolt?
The torque is defined by the fastening system documentation for the bolt grade, diameter and coating. In the absence of documentation, use the torque tables for the property class and lubricated threads, and verify with a torque wrench during installation.