## 1 Introduction  
Given their low energy consumption compared to other means of land transport, railways are gaining more and more importance in a world that urges both sustainability and economic growth [1]. This advancement of railways has resulted in an intense technological effort towards increasing both speed and load per axle of the vehicles. In parallel with this technological evolution, there are also safety issues involving the structural integrity of both vehicles and the railway infrastructure itself, which must be monitored more frequently to guarantee adequate levels of safety [2; 3; 4]. When it comes to track and vehicle condition monitoring, two possible approaches regarding sensor placement are possible, that is, installing sensors on the trackside or on the vehicle side. Track-mounted sensors can provide detailed information regarding track conditions; however, due to the lengthy extension of railway lines, it is usually only feasible to instrument small track sections, limiting this application only to very critical parts of the track. On the other hand, this approach is particularly interesting when it comes to vehicle monitoring since a small instrumented track section may be used to monitor all the passing vehicles [2; 5]. Another option to consider is the installation of onboard monitoring systems on railway vehicles. While instrumenting the vehicles directly may not be as cost-effective as installing systems on the track, it offers greater accuracy in detecting vehicle damage. Furthermore, besides being expensive, having sensors on every vehicle is not as economically prohibitive as instrumenting an entire railway track. On the other hand, this approach has a significant economic advantage when it comes to monitoring railway infrastructure. A single instrumented vehicle can provide information on a large track extension, making it a cost-effective solution for monitoring large railway stretches [6; 7]. This particular approach is usually known as drive-by, vehicle scanning, or indirect monitoring and was first introduced in 2004 by [PERSON] et al. [8] for assessment of a bridge's natural frequency from the vehicle's response only. In recent years, many researchers have concentrated their efforts on developing methodologies for detecting damage to transport infrastructure based on data collected by instrumented vehicles. Concerning railway applications, significant advancements were achieved in detecting damage both on the track and in vehicles while, unlike road applications, only a few works involving drive-by methodologies for condition assessment of railway bridges have been developed so far. This article focuses on presenting the current state-of-the-art on drive-by methodologies applied to railway subsystems. Given the extent of the subject matter constituted by the set of these applications, the authors have decided to divide the discussion into two articles. As mentioned in Part I [9] of this work, which is related to drive-by railway bridges' condition assessment, this second part covers works related to track and vehicle condition evaluation through indirect monitoring methodologies via instrumented trains. Altogether, the two papers comprise applications of drive-by methodologies applied to the main railway infrastructure subsystems, contributing to the existing field of knowledge. Table 1 summarizes the available literature review articles highlighting the subjects discussed in comparison with those covered by this work. A discussion of the content of these papers is presented in Part I [9]. As can be seen, there is a clear gap in the related literature when it comes to works dedicated to the application of drive-by methodologies to railway subsystems. This is especially true when it comes to bridges, where most of the reviews are focused on highway applications, and only a few studies related to the railway are cited.