Keywords: track vibrations, very high-speed railways, dynamic model, experimental measurements, accelerations.

Some new high-speed lines currently under construction are expected to enable maximum speeds of 350 km/h on commercial operation. The consequences of travelling at such high speeds may present some concerns in terms of track geometry deterioration, and thus in track maintenance costs, due to the known increase of track vibration levels. In fact, it is known that at very high speeds (>300 km/h), ballast vibrations lead to ballast deterioration and, consequently, to important settlements and changes in track geometric quality. This paper presents a review on the existing literature on real measurements of track accelerations and on the corresponding admissible maximum values.

Within this scope and concern, it is essential to have a dynamic numerical model consistently validated with real experimental measurements in order to understand the dynamical behaviour of the track for very high speeds and to enable the prediction of vibrations induced in the track.

Some simulations carried out with a dynamic track model (validated with experimental measurements made in high-speed tracks in France and Spain) in order to evaluate the influence of increasing train speeds in the level of vibrations reached in different types of tracks are described. Comparative analysis to find improved track dynamic behaviour at very high speeds contrasts the effects of different track design solutions, such as the introduction of: softer railpads, under sleeper pads, ballast mats or bituminous subballast.

Finally, critical analysis on the results obtained enable some recommendations to be drawn on the improvements to be made to ballasted track design for very high speeds running.

From the results obtained it could be deduced that, as initially suspected, both the increasing elasticity in the superstructure and increasing stiffness in the substructure had positive effects in the reduction of ballast accelerations. Also, that the farther away the insertion of anti-vibrating elements from the excitation source, the less effective is this measure. In fact, this was demonstrated in the case of using under ballast mats, which were revealed to only reduce vibrations inside the track under the ballast layer, while simultaneously amplifying sleeper and ballast accelerations. On the contrary, design solutions such as softening the railpads or introducing elastic pads under the sleepers were confirmed to be very effective solutions when it comes to diminishing the ballast accelerations and dissipating the energy of vibration. Furthermore, using bituminous sub-ballast as an alternative to granular sub-ballast is also revealed to be a very effective measure to decrease accelerations in the track and improve its dynamic behaviour at very high speeds.

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