## 1 Introduction  
In high-speed and urban rail projects, vibration-reduction measures must be taken into account for areas that are highly sensitive to vibrations such as residential areas and precision instrument laboratories. To this end, different types of resilient tracks [1; 2] and elastomer layers [3; 4] have been widely used. At the same time, vibration reduction measures can also be taken in the building. [PERSON] [5] researched the application of additional dampers which will absorb part of the energy supplied to the building to reduce vibration. In addition to the transmission to the surrounding environment, the reduction of vehicle-track vibrations also has a strong influence on the maintenance of the tracks and on their life cycle costs, being to the subject several research works in the past years [6; 7; 8; 9; 10; 11; 12; 13; 14]. Because of the high cost of and technical issues associated with the new ballastless tracks, the traditional ballasted railway tracks are still deployed extensively throughout the world today. For ballasted tracks, ballast mats have been considered as an effective solution to reduce train-induced vibrations [15]. Figure 1 shows the cross-section of a ballasted track with a ballast mat, and Figure 2 shows the installation of a polyurethane ballast mat in China. [PERSON] [16] proposed a new material which was validated to produce anti-vibration mats for railway applications, and the vibration isolation effect has been analysed through a finite element model. [PERSON] [17] has compared the track deformation and wave propagation in the soil foundation with the unisolated and isolated ballast tracks through a frequency domain model. This theoretical research mainly focused on the vibration transmission and vibration reduction of ballast mats either in the time or in the frequency domain, as well as on the dynamic behaviours of vibrations with frequencies lower than 200 Hz. However, only few studies have focused on the dynamic characteristics of track components after the installation of ballast mats and especially on the response of the track vibration in the mid- and high-frequency ranges up to several thousand hertz. Track vibration control is crucial because tracks deteriorate with increasing vibration levels, changes in track geometry and the fatigue failure of components [18]. Furthermore, tracks undergo permanent deformation and degradation, and the breakage of ballast particles is highly influenced by the external loading frequency [19; 20]. It is necessary to evaluate the vibration-reduction efficiency when the rail lines are close to sensitive areas. [PERSON] [21] has investigated the vibration isolation efficiency using a vehicle-track coupled model to predict the force transmitted through the ballast mat and then use it as the excitation to predict the building vibration nearby. However, sometimes when the line is not in operation or no experimental train is permitted to run through, it will be impossible to measure the vibration-reduction efficiency until the line is under operation. By then, it is usually difficult and costly to implement additional vibration reduction measures when it is observed that the vibration mitigation is insufficient. Under such circumstances, the wheelset drop test is often used to evaluate the dynamic characteristics of track structures. It has been observed that the structural dynamic response of the track in a wheelset drop test is equivalent to that under train operations [22]. So, the wheelset drop tests can produce reliable prediction on the vibration reduction efficiency. In a typical wheelset drop test, the wheelset is first lifted to some height (usually 20 or 25 mm) above the rail surface and then dropped. The impact force has enough energy to excite vibration in all the frequency ranges of interest. Then, the vibrations of each track component can be measured and used to analyse the vibration transfer function and vibration reduction.