Keywords: track deterioration, track degradation, energy, transition, dynamic stiffness.

A railway track, as a predominantly one-dimensionally oriented structure, exhibits longitudinal variations in system properties, on many scales, in many forms and with different degrees of intensity. They occur as a result of changes in track configuration and geometry, or in geometrical or constitutive properties of components that are present in the stress path of the load through the structure. Examples are the presence of sleeper bays, level crossings, bridges, tunnels, abutments, culverts, switch panels and foundation stiffness variations of the soil. For the convective response field under the axles of a running train these system property variations imply a non-stationary, continuously varying regime. Transitions are therefore locations of amplification of stresses and mechanical energy in the track. This energy is partly eradiated to the environment in the form of noise and vibrations (radiation damping), and partly dissipated in the track components, in the form of an irreversible part of the strain path (material damping). Because this phenomenon is cyclic, it is responsible for long-term deterioration, which is therefore always a locally induced process.

As transitions are present in the track in so many forms, this energy loss into the track is a continuous and fluctuating function of time and position along the track. This effect of continuous radiation damping starting from the wheel-rail contact, could be called the 'dynamic drag' of the running train, in comparison with other drag types that influence the energy consumption of the running train. The problem is especially relevant on soft soils with high ground water tables, where the track structure may behave as an elastic wave guide, and disruptions cause reflection and diffraction of travelling wave fields.

The dynamic track stiffness, which includes not only constitutive properties and spatial dimensions, but also the time dimension in its description of the physics of the track, proves an excellent tool to characterise the response of a railway track to train loading and the role of transitions in both the short and the long term. In the paper different theoretical definitions and practical implementations of this key concept are discussed. All forms of system damping (radiation and material damping) are combined in the imaginary part of this stiffness, whereas the real part describes the quantitative relationship between loading and deformation. Measurement of this stiffness may be a useful tool for the prediction or assessment of vibration hindrance of a railway line to the environment.

In the paper, different design measures are proposed and evaluated from a dynamic viewpoint, including the effect on the long-term degradation and the maintenance regime of the track. These include the application of elastic undersleeperpads and geogrids, as well as the design of civil structures on tracks that are prone to waveguide behaviour.

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