side chain
On some railways, a chain fitted on both sides of a centre coupler or buffer at each end of a rail vehicle, which has a hook for connection to a corresponding chain on the next vehicle in a train.
side chain: backup coupler safety line on rail vehicles
A side chain is a secondary coupling device fitted to the sides of a rail vehicle at its ends, running parallel to the main centre coupler or buffer. It consists of a chain with a hook at each end, designed to connect adjacent vehicles in a train. While the centre coupler handles the primary pulling and compression forces during normal operation, the side chain serves as a mechanical safeguard: if the main coupler fails or becomes uncoupled during transit, the side chain prevents the vehicles from separating entirely and becoming a hazard.
The side chain typically has a working length of around 1 to 1.5 metres when slack, allowing normal articulation between vehicles while remaining taut enough to catch them if the primary coupling fails. It is usually made of mild steel chain with a diameter of 10 to 16 millimetres, fitted with forged steel hooks crimped or welded to the chain ends. The hooks engage over lugs or eyes bolted to the vehicle frame, one on each side of the coupling face.
Side chains are most commonly found on older or heritage railway stock, and on some freight wagons and shunting vehicles. Modern passenger stock often relies entirely on automatic couplers such as Scharfenberg or coupler types that incorporate their own fail-safe mechanisms, making separate side chains redundant. However, freight and industrial railways in several countries still mandate side chains as a legal safety requirement, particularly where vehicles are frequently shunted or coupled manually.
The main failure mode is corrosion and embrittlement. Chains exposed to weather, salt spray, or industrial atmospheres can weaken over time if not regularly inspected and oiled. A corroded link may snap under shock load during coupling, negating the safety function. Hooks can also bend or shear if a coupling failure occurs under heavy tension, or if the chain has caught on lineside equipment during transit.
The term side chain distinguishes this device from the main chain or centre chain that forms part of the primary coupler assembly. On some railways the device is called a safety chain or coupling safety chain. The naming convention reflects its position (to the side of the main coupling) and its function (preventing complete separation). It represents an era of rail practice when mechanical redundancy was preferred to reliance on a single complex coupling mechanism.