## 3 Application of Drive-by Methodologies on the Bridge Subsystem
### Bridge Damage Identification
#### 3.2.2 Feature Extraction
While no field testing has been conducted, the scour indicator has performed well in the two evaluated numerical models, which incorporated measurement errors and train-bridge interaction effects. [PERSON] et al. [70] investigated the dynamic response of a train crossing a bridge to identify structural damage. A moving window root mean square (RMS) damage diagnosis technique is used to analyze the spectral components of axle box and bogies accelerations from healthy and damaged bridges. A damage localization error index (DLE) is introduced to quantify the accuracy of damage localization using sensors deployed in different train positions. Numerical simulations of the train-track-bridge interaction compared scenarios with varying bridge span lengths, damage levels, damage locations, train speeds, and bogie arrangement types. The magnitude of the acceleration to be measured to distinguish between a healthy and a damaged structure (i.e., the damage component) and the accuracy of damage localization are used to evaluate the proposed approach. The strategy worked well for localizing anomalies in commuter train velocity ranges. However, the damage component to be measured is in the order of \(10^{-2}\) or \(10^{-3}\) m/s\({}^{2}\), which is a challenge for implementation in a real railway scenario where track imperfections might reduce the signal-to-noise ratio. The carried-out analyses implied that the acceleration recorded on the leading bogie is best for damage identification, and the algorithm assessing the quasi-static response of the bridge performs better, enabling more precise damage localization and reduced sensitivity to track irregularities. The shortest bridges result in the greatest amounts of damaged components. The damage component increases with train speed, up to 140 km/h, and with damage level, although the best localization accuracy is achieved at lower train speeds and damage levels. A novel method for indirectly monitoring the structural health of railway bridges was proposed by [PERSON] et al. [71], in which direct integration techniques are applied to determine the bridge's apparent profile by an optimization procedure, directly from the vehicle accelerations. Using the apparent profile, Bridge Displacement Profile Difference (BDPD) is calculated and used as an indicator of temperature change and bridge damage. The frequency of BDPD is demonstrated to be effective through both numerical simulations and experimental studies on a laboratory-scale railway bridge model. The authors show that their method can accurately detect and quantify changes in the structural response of the bridge due to damage or deterioration and can provide useful information for the maintenance and repair of the bridge. A railway damage detection approach was presented by [PERSON] et al. [72] for calculating the flexural rigidity of bridges (FR) by measuring the deflections of the bridge using a drive-by method. The method involves using a model of a vehicle equipped with a high-speed laser scanning system (Traffic Speed Deflectometer and Doppler Vibrometers at eight different locations) to measure the deflection of the bridge as the vehicle crosses it. The relation between FR and deflection is given by the Unit Load Method, which is used to extract a damage index with the concept of Ratio of Spatial Standard Deviation (SSTD), i.e., spatial variability in calculated FR. The Blackman filter is also applied to regularize the system of equations as the bending moment matrix relating deflections to FR is ill-conditioned and sensitive to measurement noise. The results of the study show that the proposed method can accurately calculate the flexural stiffness of the bridge, and the calculated results are consistent with the results obtained using traditional inspection methods. In conclusion, the authors suggest that their proposed method has the potential to be a reliable and cost-effective tool for calculating the flexural stiffness of bridges, especially in situations where traditional inspection methods are not feasible.