###### Abstract  
The topic of the paper is part of the research area in which the methods of ride comfort evaluation are investigated, to identify their advantages and limitations, the correlation of the comfort indices obtained by different methods, and even the improvement of these methods as an application methodology or as an interpretation. In this paper, the Sperling index method is analyzed from a new perspective, specifically, the effect that the frequency weighting functions specific to this method have on the dynamic response of the railway vehicle carbody, as evaluated by the power spectral density (PSD) of the acceleration. The analysis is based on numerical simulation results for three different cases: respectively, the unweighted PSD of the acceleration, the frequency-weighted PSD of the acceleration with the function for the evaluation of ride quality (RQ)and the weighting function for the evaluation of ride comfort (RC). The applications for the numerical simulation are based on an original vehicle-track system model with 15 degrees of freedom, which includes a rigid-flexible coupled model of the vehicle and the equivalent model of the track. This model considers important elements that can influence the vertical vibration behavior of the carbody, namely, the carbody structural flexibility, which is represented by an equivalent Euler-Bernoulli beam, the system through which the longitudinal forces are transmitted from the bogies to the carbody, and the elasticity of the wheel-rail contact. Based on the analyses developed in the paper, the relevant conclusions regarding the effect of the frequency weighting functions specific to the Sperling index method on the dominant vertical vibration modes of the carbody are synthesized. These conclusions are correlated with the results of the Wz index for the RQ evaluation and the RC evaluation. r 2025 15 507 1100 1100 1100 1100 1100 1100 1100 1100  
railway vehicles in terms of dynamic behavior are included in the railway application standard EN 14363 [2]. For the evaluation of RC, no universally valid method can be applied at the international level, because, from one country to another, the vibration characteristics of railway vehicles may vary due to the condition of the vehicles and the lines or the operating conditions [3]. Also, the methods for RC evaluation must be adapted to the specifics of the traffic, namely urban, suburban, or long-distance. For example, in Japan and Korea, where trains in urban areas are very crowded, it is a common practice that the permissible limits of vibration to which passengers are exposed are not established for the seated position but for the least comfortable position [4; 5]. This explains why there are several methods for evaluating RC that have appeared over time, independently, in different countries [6]. Out of the practical need to assess the state of comfort that passengers feel during the train journey, several standards, guidelines, and procedures have been developed. The AS 2670 standard [7; 8], the BS 6841 standard [9], the UIC 513R [10], the ISO 10056 standard [11], and the railway applications standard EN 12299 [12] are based on different versions of the ISO 2631 standard developed by the International Organization for Standardization [13; 14; 15]. A reference method for evaluating RQ and RC of the railway vehicles is the method known as the Sperling index method or Wz index method [16; 17]. This method introduces, for the first time, the notions of the RC index and the RQ index, the latter also being referred to asthe Wz index. In the context of the considerations presented above, several researchers have come to the conclusion that, in the absence of a generally applicable RC evaluation method, it would be useful to establish a relationship between comfort indices obtained by different methods, which would allow the immediate transformation of one index into another, or, at least, to identify the advantages and limitations of one RC evaluation method in relation to another.