# Indirect Assessment of Railway Infrastructure Anomalies Based on Passenger Comfort Criteria
## 2 Ballasted Track  
Worldwide, as well as in Portugal, the most common railway system is the ballasted track system. Ballasted track system has lower construction costs and adequate responses to static and dynamic forces [28]. This track type is grouped into two main categories: superstructure and substructure. The former consists of rails, sleepers, and ballasts and components that connect these elements. In contrast, the latter is associated with the geotechnical system and comprises sub-ballasts, embankments, and subgrade layers [20; 21; 29; 30]. Figure 1 presents a schematic representation of the railway ballasted track system. Track integrity problems are dependent on the superstructure and substructure deformation. Both are related, but the health of the track substructure defines the structural performance [21; 30]. The main substructure issues are related to sub-ballast and subgrade deterioration, which can induce poor drainage and track settlements. Those issues may also reduce track stability, capacity, and safety, leading to misalignments and increased wear [21]. Superstructure track faults can be grouped into three main conditions: those related to geometry (cross-level, alignment, longitudinal levelling, twist, and gauge), those dependent on rail surface faults (surface, corrugation, fatigue cracking, squat, and creep), and, lastly, those dependent on the ballast conditions (fouled ballast, ballast pockets, and poor ballast drainage) [20; 28]. Track defects and irregularities negatively affect track performance and safety. Wavelengths between 10-120 m mainly influence passengers' comfort. EN 13848 [31] defines three wavelength intervals for the evaluation of vertical and lateral track geometry conditions: D1 (3-25 m), D2 (25-70 m), and D3 (70-150 m). The D1 wavelength range ir  
Figure 1: Schematic illustration of railway ballasted infrastructure. regularities are mainly associated with running safety conditions, whereas D2 and D3 ranges strongly affect ride comfort [31; 32]. Lower wavelengths define rail surface faults; for example, rail corrugation can appear between 0.03-0.08 m of wavelength, whereas rail squat comprehends 0.02-0.04 m of wavelength. A discontinuity in the rail caused by a weld or joint can present 0.01-0.02 m of wavelength [33; 34]. It should be highlighted that the aforementioned defects' wavelengths are the most typical ones. Nevertheless, depending on its severity and dimension, the same type of defect can produce higher or lower wavelengths. Figure 2 presents the rail track geometry conditions evaluation defined by EN 13848, along with rail surface faults (rail corrugation, squats, and discontinuities) and passenger comfort wavelengths. Rail defects and increased speed further amplify the rail vehicle-track dynamic interaction forces [19; 20; 30; 35]. When a rail vehicle and track do not present defects or abnormalities, the vehicle exerts low-frequency forces (under 20 Hz). However, that frequency increases in the presence of faults, leading to dynamic impact wheel-rail forces. Moreover, dynamic loads, which are connected to track irregularities, amplify the rail deterioration rate. Vehicle vibration magnitudes are strongly linked with track irregularities and can be used to assess general track conditions effectively [19; 29; 30; 36]. [PERSON] et al. [37] and [PERSON] [38] noticed that railway track defects could be identified based on acceleration records according to their peak values, as illustrated in Figure 3.  
Besides affecting comfort, track irregularities can lead to derailment, which can have significant consequences [18]. Therefore, promoting scheduled railway infrastructure maintenance is crucial to maintain high safety levels and the comfort of passengers. When a railway vehicle passes over a railway track, it causes static (weight) and dynamic (inertia and impact) loads. That dynamic load leads the railway track to vibrate for a certain period and, consequently, deform.