# Evaluation of Measurement Strategy for Track Side Monitoring of Railway Wheels
## 1 Introduction
Assessment of the present condition and forecasting of the future condition of the wheelset strongly relies on the measurement stage, and hence selecting an adequate sensor type for measurement purposes is crucial. For railway wheelset monitoring, the data acquisition approach can be broadly classified into two types of measurement: in-workshop measurement, when the wheels are idle, and in-service measurement, when the wheels are in motion. In-service measurements can be further divided into two types based on the position of the data acquisition sensor: onboard measurements and wayside measurements. Ultrasonic techniques [20; 21], infrared camera [22], and magnetic techniques [23; 24; 25] are commonly used monitoring tools used during in-workshop measurements of rail wheels. Acoustic techniques [26; 27], ultrasonic technique [28], vibration measurements [29; 30], and magnetic techniques [31; 32] are common types of monitoring techniques used for in-service and onboard inspections of rail wheels. The in-service and wayside inspection of rail wheels commonly makes use of monitoring systems such as strain gauges [33; 34], Fibre Bragg grating sensors [35; 36], ultrasonic techniques [37; 38], vibration technique [39; 40], acoustic technique [41], and laser-air hybrid ultrasonic techniques [42]. Most of the studies using the abovementioned techniques have been carried out to detect wheel flats, surface defects, and subsurface defects. However, abnormalities on the profiles of the wheel such as asymmetrically worn wheel and hollow wear are understudied [6; 43]. Train wheelsets are subjected to constant wear mainly due to the friction arising during the contact between the wheel and the rail. Once the wear on the wheelset exceeds the safety limit, the wheelsets need to be reprofiled or replaced, otherwise normal wheel-rail interaction will be restricted and can ultimately lead to derailment. Further, asymmetrically worn and hollow-worn wheels can also lead to significant damages on the rail head and in switches and crossings, thus increasing the time and cost for maintenance activities. Hence timely assessment of the wheel profile using suitable condition monitoring techniques is crucial for infrastructure managers to ensure safety and high quality of service. A general criticism of measurement systems, especially the ones used for condition monitoring, is that they do not always produce reliable data [19]. Instead, they can give measurement values with huge discrepancies from the actual value due to errors or to unreliable measurement patterns [44]. Such a scenario can lead to incorrect assessment resulting in unwanted interventions and wrong or no actions being taken, thus increasing the risk of catastrophic failures and adding to the cost paid by the infrastructure owners. Thus, it is crucial to ensure that the measured data are reliable and of high quality, since the management decisions based on the information derived from the data are only as decisive as the data themselves. Wheel profiles are usually inspected and measured in a workshop environment using a handheld measurement device, measuring the wheel profile at one position of a non-rotating wheel. Over the past decade, the wayside monitoring of wheel profiles has gained significant importance for monitoring wheel profiles [4; 13]. During the wayside inspection, the wayside equipment measures the passing train, resulting in a one-slice measurement strategy [4; 6; 18]. Typically, during wheel profile inspection (both in-workshop and wayside inspection) the profile is measured only at a specific position of the wheel, the so- called 'one-slice' approach. This 'one-slice' approach is carried out on the basis of the assumption that the wheel profile has the same shape independent of the measurement position along the circumference of the wheel. However, to ensure that reliable information is derived from the measurements, multiple measurements need to be conducted to ensure a reliable representative measure of the wheel profile along the circumference of the wheel.