Wheel & axle monitoring

Wheel and axle monitoring is the detection of defects in railway wheelsets and axle box bearings, using trackside sensor installations or instruments mounted directly on rolling stock, to prevent unplanned failures and derailment risk.

A railway wheelset — two wheels and a shared axle — is subjected to high cyclic loading in service and to progressive surface damage. Wheel defects including flat spots, polygonisation and shelling generate periodic impact forces that accelerate deterioration of track and rolling stock, and create derailment risk if not detected before they develop beyond safe limits.

Axle box bearing failure is a leading cause of unplanned freight and passenger service disruptions across European networks.

Monitoring is deployed in two configurations: wayside systems detect conditions as a train passes a fixed trackside location; onboard systems monitor continuously throughout a journey.

Wayside monitoring

Hot Axle Box Detectors (HABDs) use infrared pyrometers positioned beside the rail to measure bearing housing temperature as trains pass at speed. Overheating indicates lubricant failure or internal component damage. HABDs are deployed at regular intervals on main lines across European networks, connected to signalling and control systems that can route trains with flagged bearings to inspection sidings.

Wheel Impact Load Detectors (WILDs) use strain gauges or load cells embedded in the rail to measure the vertical wheel-rail contact force as each wheel passes. Force peaks above baseline indicate flat spots and other surface defects. The ranked output is used to prioritise inspection across a fleet.

Wheel Profile Measurement Systems use laser scanners to measure the cross-sectional geometry of passing wheels, identifying profiles — flange height, flange thickness, tread radius — that have moved outside maintenance limits.

Acoustic Bearing Detectors position sensitive microphones at rail level to capture high-frequency signatures from passing bearing assemblies. Internal bearing damage produces identifiable acoustic patterns detectable before temperatures have begun to rise, providing earlier warning than thermal detection alone.

Onboard monitoring

Accelerometers mounted on axle boxes detect vibration signatures of wheel surface defects and bearing faults. Flat spots and polygonisation generate periodic impact signals at harmonics of wheel rotation frequency. Onboard systems process these signals continuously and can transmit diagnostic data via telematics to fleet management, flagging developing defects between wayside detection points.

Regulatory context

The Technical Specifications for Interoperability for rolling stock — the LOC&PAS TSI and the WAG TSI — set maintenance limit values for wheelset geometry and define the conditions under which rolling stock must be withdrawn from service. European operators are required to record wheel and bearing condition and demonstrate that maintenance intervals satisfy safety case requirements.