## 2 Field Measurements  
### Overview  
In this study, field measurements were performed to quantify the dynamic responses of direct fixation tracks to different train types (Airport Railroad Express (AREX) and Korea Train Express (KTX)) according to rail pad replacement. To experimentally evaluate the dynamic behaviors of the existing direct fixation track of the Yeongjong Grand Bridge, field measurements of the operational track were conducted. The location of the measurement was selected as the direct fixation track inside the truss bridge, as shown in Figure 3a,b. The vehicles operating in the measurement section were AREX (normal train) and KTX (high-speed train), which have different operational velocities and wheel loads. In this study, the dynamic responses of the track, which are generated while trains are running, were used for analysis. The properties of the trains running in the measurement section are presented in Table 1.  
\begin{table}
\begin{tabular}{l l l} \hline \hline  & **AREX** & **KTX** \\ \hline Train composition & Tc1-M-M\({}^{\prime}\)-T1-M\({}^{\prime}\)-Tc2 & P-M-16R-M-P \\ Total length of train (m) & 120.90 & 388.10 \\ Axle distance (mm) & 2200 & 3000 \\ Wheel diameter (mm) & 860 & 920 \\ Maximum speed (km/h) & 120 & 330 \\ Operational speed (km/h) & 73–75 & 61–98 \\ Effective beating distance (m) & 13.8 & 18.7 \\ Designed wheel load (kN) & 50 & 85 \\ \hline \hline \end{tabular} Note: Korea Train Express (KTX) = P (power car); M = motorized trailer; R = passenger car; Airport Railroad Express (AREX) = Tc (control car); M = motorized power car; T1, T2 = passenger cars.  
\end{table}
Table 1: Properties of the tested train.  
The dynamic wheel load and lateral wheel load for each position of the track girder including the support, the quarter, and center of the span, as well as the bending stress of the rail, the vertical acceleration of the rail and track girder, the vertical and lateral displacement of the rail, and the vertical displacement of track girder were measured, as shown in Figure 3a. Figure 3b shows the sensors installed at the site. The vertical wheel loads and lateral wheel loads were measured using a wheel-rail contact force sensor (i.e., shear strain gages coupled to a full Wheatstone bridge circuit [2,4,14]). The strain gage bridges were calibrated using a hydraulic ram and a load cell to obtain measurements with an accuracy of 2% [2,4,14]. The shear strain gages were attached to both rails between two consecutive rail supporting points [2,4]. The bending strain of the rail was measured by longitudinally attaching a one-axis strain gage to the bottom flange of the rail, as shown in Figure 3[2]. The strain gage was placed between rail supporting points (i.e., a place of rail fastening system) at the center [2].  
To prevent data distortion and loss, the sampling rate was set to 1 kHz [2,4,14]. The vertical rail and track girder displacements were measured using displacement transducers mounted on a jig anchored at the side bridge inspection passage, which was not affected by the passing train, as shown in Figure 3[2]. To analyze the effects of vibrations generated by passing trains on the direct fixation rail and supporting girder, the track vibrations were analyzed before and after the replacement of the rail pads using accelerometers installed on the rail and girder as shown in Figure 3b. The accelerometers were installed in the vertical direction on the top surface of the rail flange and on the upper flange of the track girder at the between-rail supporting points (i.e., rail fastening system locations).  
Figure 3: Sensor installation. (**a**) Location of the measurement and (**b**) vertical and lateral wheel load strain gages, strain gages, vertical rail, and track girder displacements measured using displacement transducers mounted on a jig anchored at the side bridge inspection passage, which is not affected by the passing train [2].