# A Hybrid Model for Freight Train Air Brake Condition Monitoring
## 1 Introduction  
European freight rail transportation is undergoing a transformative shift towards digitalisation, driven by the need for greater safety, reliability and operational efficiency [1, 2]. The TRAN54M-R, for example, is a significant European initiative aimed at making rail freight the backbone of a low-emission and resilient transportation network [3, 4]. The digitalisation of freight train operations can be achieved by integrating advanced sensor technologies for collecting critical operational data, Condition Monitoring (CM) systems for enabling the detection of potential faults or anomalies [5, 6], and Condition-based Maintenance (CBM) systems for optimising maintenance and scheduling interventions. The implementation of real-time onboard monitoring systems has been extensively reviewed in the literature [7, 8, 9], focusing on wireless sensor networks and energy-efficient monitoring solutions [10, 11, 12] due to the absence of power availability onboard freight trains. Moreover, using models and diagnostics algorithms, it is possible to identify malfunctions in the system and communicate them to a remote-control unit [13, 14]. This approach has a huge impact on improving the safety of the convoy, scheduling timely inspections and reducing the cost of maintenance and the number of wagons taken out during service. The air brake, in particular, is one of the most important and complex systems in railway vehicles. Moreover, compared to the other subsystems of the convoy, the air brake is one of the major components subjected to malfunctions: of the 500 freight rail derailments that occur in Europe each year, approximately 10% are caused by issues related to brake failure [15]. The main causes of railway brake system failure include mechanicalissues and malfunctions, operational mistakes and insufficient maintenance procedures [16]. Moreover, due to its complexity and high compactness, it is often difficult to make a precise diagnosis and identify the malfunction's root causes. The brake system malfunction is one of the main causes of railway accidents and can have serious repercussions, including service interruptions, derailments, and even fatalities. An example can be found in the runway and derailments of wagons at Ashbury in 2010 [17]. It has been found that the immediate cause of the derailment of five loaded wagons was that the handbrakes were defective due to the wrong adjustment of the handbrake force-limiting bolts. This case illustrates how mechanical failures, coupled with insufficient maintenance protocols, can play a crucial role in triggering catastrophic accidents. A summary report and database of derailment incidents are reported in [18]. It is shown that a key derailment cause in France includes brakes not properly checked or tested, while Great Britain has a higher percentage of derailments as a result of brake failure, the majority of which due to handbrake left applied to wagons when moving on the mainline. An analysis of the causes of failures in the brake equipment of the rolling stock of the railway of Uzbekistan is presented in [19]: it is shown that the main reasons for the withdrawal from the operation of brake pads include air leakage in the distributor and violation of the integrity of the brake pipe due to low tightness of the threaded connection. An efficient model of the air brake system can be used for malfunction detection during normal service but can also be employed to enhance monitoring effectiveness and minimise necessary resource investments during brake inspections. Current brake inspection procedures require technical operators who manually check the health status of the components within the system: these procedures are time-consuming, wasteful of human power, costly, and not suitable for detecting certain types of failure [20]. Often, inspectors are employed to walk or drive the length of a stationary train, looking for these faults and checking the application to the wheel of the individual brake blocks by visual inspection.