Rail: Wheel and axle monitoring
Wheel and axle monitoring is the detection of defects in wheelsets and axle box bearings, using trackside installations or onboard instruments, 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.
Defect types and detection strategy
Wheel defects including flat spots, polygonisation and shelling generate periodic impact forces that accelerate deterioration of track and rolling stock. They create derailment risk if not detected before they develop beyond safe limits.
Axle box bearing failure is a recurring cause of unplanned freight and passenger service disruptions across European networks. Within sensors and measurement, monitoring is deployed in two configurations: wayside systems detect conditions as a train passes a fixed trackside location, while 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. They are 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, and the ranked output is used to prioritise inspection across a fleet.
Profile and acoustic detection
Wheel Profile Measurement Systems use laser scanners to measure the cross-sectional geometry of passing wheels. They identify 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. Developing defects are flagged between wayside detection points.
Regulatory context
The Technical Specifications for Interoperability for rolling stock — the LOC&PAS TSI (locomotives and passenger rolling stock) and the WAG TSI (freight wagons) — set maintenance limit values for wheelset and wheel geometry, including minimum flange thickness relative to wheel diameter, to be entered as in-service limits in each vehicle’s maintenance file.
The conditions under which a vehicle is withdrawn from service follow from that maintenance file, under the responsibility of the vehicle’s Entity in Charge of Maintenance (ECM). Commission Implementing Regulation (EU) 2019/779 requires the ECM — not necessarily the railway undertaking operating the vehicle — to record wheel and bearing condition and to demonstrate that maintenance intervals are consistent with its certified maintenance system.