Firstly as the wheel ndash rail interface is the

Fig. 2. Quasi-static track deflection.Figure optionsDownload full-size imageDownload as PowerPoint slide
In contrast, dynamic excitation is speed dependent and arises from factors such as changes in stiffness due to sleeper placement, irregularities at the wheel/rail interface and the RI-1 support conditions. Dynamic excitation from irregularities and rail joints is slowly becoming a less influential excitation mechanism due to the widespread use of continuously welded rails and improved track maintenance.
2.2. Wave propagation
Upon generation, the vibrations caused at the wheel/rail interface propagate through the track and into the free field in the form of waves. These waves are categorised as either body waves or surface waves. Surface waves travel along a structures (i.e. soil) surface and decay exponentially with depth. Body waves propagate primarily beneath the soil surface. Wave propagation characteristics are shown in Fig. 3.
Fig. 3. Seismic wave propagation (slice view – after [18]).Figure optionsDownload full-size imageDownload as PowerPoint slide