## 2 Theoretical Discussion
### Effect of Different Track Conditions and Types on the Parametric Evaluation  
As is shown in the above discussions of the parameters, the effect of the dynamic load is a factor for consideration in all of the calculations. The spring coefficient that comprises the track modulus denotes the supporting strength of the track, and is measured by the factor of wheel load during track side measurement. In most cases for the trackside measurement of TSS, Hooke's law is used, in consideration that the track is considered as a beam element to calculate the track support stiffness. The TSS is derived using the variables of maximum dynamic wheel load during trackside measurement, and the maximum rail displacement (deflection). In this regard, [PERSON] compares the TSS between ballasted and concrete slab tracks [16]. Based on the measured wheel load and vertical displacement of ballasted and concrete slab tracks, the averaged TSS values of 152.67 kN/mm for the ballasted track (Figure 3a) and 43.03 kN/mm for concrete tracks (Figure 3b) are derived (Refer to Figure 3 for the comparison). This is a measurement of the track support stiffness at the wheel-rail contact point, therefore, it is reasonable to consider that while the overall track modulus for the concrete track should be higher in theory, these values can be seen to be reasonable only when considering that the fastener spring coefficient of the respective track types would affect the measurement results. Ballasted tracks employ high spring coefficient fasteners and rail pads to compensate for the softer ballast layer, and concrete tracks use rail fastener and pad types with low spring coefficients to counteract the effect of the concrete layer's high stiffness. Other similar studies in Korea indicate that the average TSS values of ballasted track structures are approximately in the range of 130-160 kN/mm, while ballast-less slab track TSS values range over a wider extent, between 60-120 kN/mm [17]. Refer to Figure 3 below for details:  
These findings and results indicate that an investigation of the respective parameters for dynamic stability evaluation is required. The problem is not the variance between the TSS obtained by  
Figure 3: Track support stiffness (TSS) comparison of ballasted and ballast-less slab tracks: (**a**) TSS of the evaluated ballasted track, and (**b**) TSS of the evaluated ballast-less slab track [17]. measurement, as it has already been well established that the behavior of the ballasted and ballast-less tracks are different ([PERSON] provides an extensive study on this subject matter) [18; 19]. The problem is that despite this difference in the properties, the evaluation parameters and criteria is the same for both ballasted and ballast-less tracks between different structure types (railway bridges, tunnel structures, earthwork tracks, etc.) of the same track types [20; 21; 22]. Furthermore, as far as the trackside measurement method is concerned there are also numerous studies and experimental results that take into account the environmental effects on the dynamic stability and the field instrumentation conditions [4; 23]. [PERSON] et al. indicates that humidity and temperature may be relevant factors when studying parameters about the wheel/rail interface friction condition, and investigates the effect of humidity and temperature on the railhead contamination on the performance of friction modifiers [24]. [PERSON] discusses that wind pressure is also a key factor that affects the hunting movement of the vehicle, particularly in either curved rail structures or in tunnels [25] and can drastically change the evaluation results of a track's measured wheel load. In open areas, rain would affect concrete and ballasted tracks differently, as ballasted tracks would be subjected to fouling and concrete tracks would be subject to cracking, which in turn would affect the dynamic performance evaluation of the track.