Keywords: ladder track, wheel-rail vibration, dynamic response, optimization method, rail corrugation.

Floating ladder rail tracks, which can significantly reduce subway track vibration and noise, have already been installed at several subway sites in Beijing. The steel rails are fixed onto successive ladder-like sections with two parallel longitudinal reinforced concrete sleepers, which are then mounted upon discrete resilient supports on a concrete bed. However, there is serious vibration and noise in some sites of the Beijing subway lines, which use the ladder track; and even rail corrugation has been generated at some sites. So the main purpose of this study is trying to solve these problems by optimizing the parameters of ladder track components.

In this paper, first measurements using a tape measure are carried out in order to clarify the characteristics of the short-pitch rail corrugation of the ladder track, which is used in the Beijing subway lines. The rail vibration responses are measured using vertical and lateral accelerometers, installed at the top of the rail. The sleeper vibration responses are obtained by vertical accelerometers, which are installed at the sleeper. Measurements are performed both at the ends of the sleepers and at the sleeper mid-span. Then the frequency domain and the time domain of the ladder track models for minimizing the ladder track vibration problem have been developed taking into account in the elastic model for the frequency analysis and the wheel-rail contact in the time domain analysis. The comparison concentrating on the rail vibration frequency has shown that the results of the simulations are in good agreement with the measurements. After this parametric study, the optimization of the ladder track parameters to minimize the vibration has been achieved using the multipoint approximation method (MAM).

Based on the experimental and theoretical studies, the following results were obtained. (1) Wheel-rail resonance occurs at 134Hz in ladder track of the Beijing subway lines, which is the main reason for noise and rail corrugation. (2) Changing the fastening parameters mainly effects the high frequency vibration characteristics of the ladder track. The sleeper mass has some effect on the frequency domain, but the effects are not obvious. Varying the resilient material parameters mainly effects the low frequency vibration characteristics of the ladder track. (3) The results of the optimizations have shown that the wheel-rail resonance can be avoided by increasing the vertical damping of the resilient material and by decreasing the vertical stiffness of the fastening. The results have also revealed that the fastening damping is not necessarily the larger the better, but it should be synthesised considering all the parameters to avoid wheel-rail resonance.

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