# Indirect Assessment of Railway Infrastructure Anomalies Based on Passenger Comfort Criteria
## 1 Introduction  
Railways are one of the most widely used public transportation systems, mainly because of their high transportation capacity, reduced boarding time, and the possibility of making better use of the journey time to work or enjoy train facilities. Additionally, due to their low environmental impact, multiple governments are promoting their use as a mass transportation mode. Following this recommendation, train passengers have continuously increased from 2013 until 2020 (the year of the COVID-19 pandemic). Between 2015 and 2019, an increase of 10% was found in Europe, and approximately 4000 billion passenger kilometres were recorded worldwide. During the COVID-19 pandemic, train passenger numbers decreased to historical levels due to multiple lockdowns [1; 2; 3]. Currently, the number of passengers is even higher than in the pre-pandemic period [4]. Increasing trains' attractivity and journey comfort will ensure the continuation of this trend. Passengers define a comfortable ride based on safety, comfort, and user conditions [5; 6]. High safety levels are guaranteed through adequate maintenance actions based on corrective or preventive strategies. The corrective action occurs after a fault is recognised and intends to recover the normal working state [7; 8; 9; 10]; in contrast, proactive measures intend to prevent and minimise eventual faults that did not yet cause the system to fail, thus anticipating the problems. The main goal of preventive maintenance is to preserve system functions and prevent rail system failure. It is performed according to a defined scheduled time. On the otherhand, condition-based maintenance (CBM), the typical railway track infrastructure condition monitoring system, is a preventive maintenance strategy that acts when there is evidence of failure. Railway track infrastructure monitoring actions can be performed in two ways, either by human or by automatic means, depending on the type and extent of the work required [11; 12; 13]. Human inspection is performed by well-trained inspectors who periodically walk along railway track infrastructure to detect defects. The inspectors can be aided by portable equipment for the auscultation and measurement of the geometric parameters of the track infrastructure. However, this maintenance technique results in high costs and can be potentially hazardous for inspectors. Moreover, human inspection can only be applied by stopping or restraining traffic, and its results highly depend on the observer's capability of detecting anomalies and recognising critical situations [7; 14]. Due to the problems associated with human inspection and advances in technology, automatic inspection methods have been developed, such as using inspection vehicles. These dedicated vehicles can detect railway track infrastructure defects and evaluate infrastructure performance. Generally, an inspection vehicle uses optical and inertial sensors linked to a global positioning system (GPS), which increases the method's efficiency and decreases the required time [8; 9; 15; 16]. This inspection is periodically performed along the railway track infrastructure. However, in addition to being expensive, these vehicles introduce traffic disruptions during the inspection, affecting regular service operations. Different types of inspection vehicles are used worldwide. In Portugal, the EM 120 inspection vehicle, owned by Infraestruturas de Portugal (IP), is responsible for identifying maintenance needs and railway track infrastructure failures. After detecting a failure, the defect is fixed and, ideally, the system is restored to its initial state [10; 12]. More recently, cargo vehicles have been used as instrumented inspection vehicles, particularly in the axle box [17; 18; 19; 20; 21; 22]. However, implementing the experimental setup is problematic for this approach, limiting its use in passenger trains. These vehicles have the advantage of continuously surveying the track without any traffic interference, thus providing information on a more regular basis at a reduced cost.