## 2 Overview of Drive-by Condition Assessment of Railway Bridges  
Indirect monitoring of bridges based on their dynamic response recorded by sensors installed on a vehicle during its passage has attracted the interest of several researchers due to the advantages it offers over direct monitoring, which involves installing instruments on the bridge itself and assessing their recordings as they evolve. Through indirect observation, we account for the fact that both the vehicle and the bridge vibrate during the crossing and that their dynamic responses are coupled as long as their contact is maintained through the vehicle wheels. Hence, the vehicle has a dual mission, both as an exciter and receiver of the bridge's vibrations when it travels over it. Making use of this dual purpose, several methods for detecting structural degradation have been developed, particularly for highway bridges. So far, however, little research has been published evaluating the feasibility of indirect monitoring approaches for railway bridges [19]. Recent technologies for monitoring the structural integrity of railway bridges include so-called drive-by damage detection [20; 21]--the concept of employing the train passing over the bridge as an excitation source and the sensors placed on the vehicle as opposed to the bridge. A limited number of commercial trains equipped with a small number of sensors might monitor the condition of a large number of bridges using the train's response to determine whether bridge damage exists. The detection methods rely primarily on comparing acceleration data obtained from instrumented trains with a reference condition corresponding to a healthy structure. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multicolumn{2}{c}{**Bridge Subsystem**} & **Railway Track** & **Railway Vehicles** \\ \cline{2-5}  & **Highway** & **Railway** & **Subsystem** & **Subsystem** \\ \hline [PERSON] et al. [8] & – & – & ✓ & ✓ \\ \hline [PERSON] et al. [9] & – & – & ✓ & – \\ \hline [PERSON] and [PERSON] [10] & ✓ & – & – & – \\ \hline [PERSON] et al. [11] & ✓ & – & – & – \\ \hline [PERSON] et al. [14] & – & – & ✓ & ✓ \\ \hline [PERSON] and [PERSON] [12] & ✓ & – & – & – \\ \hline [PERSON] et al. [15] & – & – & – & ✓ \\ \hline [PERSON] et al. [13] & ✓ & – & ✓ & – \\ \hline [PERSON] et al. [16] & ✓ & – & ✓ & – \\ \hline [PERSON] et al. [17] & ✓ & – & – & – \\ \cline{2-5}
**Present work** & **Part I** & – & ✓ & – & – \\ \cline{2-5}  & **Part II** & – & – & ✓ & ✓ \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main topics addressed by the state-of-the-art review articles. The primary idea behind these techniques is that structural damage modifies the mechanical properties of the bridge and, therefore, the dynamic behavior of the vehicle interacting with it [22]. Then, the condition of the bridge may be determined based on the dynamic responses detected on board the train, such as on the axle box, bogie frame, and carbody. According to [PERSON] et al. [22], the primary concern regarding the practical applicability of the drive-by method is that the train's response may be affected not only by the condition of the bridge but also by factors unrelated to the damage, such as the level of track irregularities, the vehicle's dynamic response to different passenger loads, and other uncertainties related to the train's speed variation and accurate positioning with respect to the bridge. Drive-by methods may be divided into two primary groups: (a) modal parameter-based techniques and (b) non-modal parameter-based techniques. The former, consisting of the identification of bridge properties such as natural frequencies [23; 24; 25; 26; 27], damping values [28; 29], and mode shapes [29; 30; 31] based on the vehicle response for most of the proposed methodologies has been demonstrated to be effective up to limited speeds (i.e., 60 km/h). These modal properties can be further used for damage assessment; however, in the first two cases, they are incapable of identifying accurately the damage location.