# Railway Track Monitoring Using Train Measurements: An Experimental Case Study
## 1 Introduction  
Railways constitute a major portion of transport infrastructure in most countries. It is essential to employ good strategies for the maintenance of railway networks to avoid a disruption to services and ensure the safety of the system. For example, the railway track condition needs to be regularly monitored to detect faults at an early stage before they become major issues. Several types of defect can happen on railway tracks: rail cracks, track settlement, landslides onto track, hanging sleepers, etc. [1]. Visual inspection or 'walking the track' is one of the most common methods of track inspection. However, it is expensive, subjective, and some faults can be missed. A track recording vehicle (TRV) is a mechanised method of railway track inspection while using optical and inertial sensors in a specialised vehicle. The inspection vehicle periodically travels over the rail network and measures several geometric parameters of the track, such as: longitudinal level of left and right rail, alignment of left and right rail, track gauge, cross level, twist, curvature and curve radius, gradient, and position using GPS [2]. However, these vehicles are expensive and they may not always be available for the whole network. In addition, they do not always travel at the same speed as normal train traffic, which, in some cases, disrupts regular services. Drive-by monitoring is an inspection method in which the responses that are measured on instrumented trains/vehicles are employed for the condition monitoring of components of transportinfrastructure, e.g., bridges [3; 4], highway pavements [5], and railway tracks [6; 7]. In the railway application, sensors are installed on an operational train to provide faster, cheaper, and more reliable monitoring approaches. It does not disrupt service provision and it can provide real time data on track condition, facilitating more timely responses to developing track failures. Such instrumentation can obviate the need for specialist vehicles, such as the TRV. In effect, it is a move from a small number of accurate measurements under controlled conditions to a large number of less accurate and less controlled measurements. Several types of sensor technology have been employed for drive-by railway track monitoring, such as: laser technology [8], camera [9; 10], and inertial sensors (accelerometers, gyrometers, etc.) [11]. Some of these technologies can be used together or separately, depending on the main purpose of monitoring. Laser optical sensors are usually considered to be expensive and they may involve a high maintenance cost. Camera-based systems are cheaper to be employed, but they require expensive image processing methods and provide limited information. Inertial sensors have been extensively used in the literature due to their simplicity, low price, and effectiveness. Inertial sensors can be installed on the train axle box, bogie, or inside the carriage, and the responses are continuously measured. These measurements have been used for several health monitoring purposes in the literature, as summarized in Table 1.  
In the context of railway track components, [PERSON] et al. [13] investigate the degradation of a railway crossing while using real train axle box measurements over time. They show that uneven deformation between the wing rail and crossing nose and local irregularity in the longitudinal slope of the crossing nose can be identified. [PERSON] et al. [14] employ axle box acceleration measurements for the monitoring of bolt tightness of rail joints. [PERSON] et al. [15] use drive-by measurements from a train axle box for condition monitoring of insulated joints. The power spectrum of the acceleration signals is used as a damage indicator. [PERSON] et al. [16] present a surveying approach for the detection and classification of track parameters, such as welded joints, turnout frogs, and squats. Changes in track typologies can be detected using measurements from a train axle box.