The success of any measuring system lies on its robustness, i.e. the capacity to cope with its mission regardless of the disturbance coming from external sources. For the system described in this paper there are several potential sources of "noise" which make the estimation of the force less reliable:

• possible irregularities in track tamping,
• dynamic behaviour (viscoelastic) of rail fastenings and ballast,
• dynamic vehicle response (coupling) when analysed together with the superstructure,
• speed variation of traffic over the monitoring station including a number of sensors.

In this paper the Flextrack tool of the VI-Rail multibody software was used in order to evaluate the effect of varying track stiffness and vehicle speed on the sleeper reaction force and, thus, total sensor signal. Multibody simulation results were used as input for a finite element model in order to evaluate the behaviour of the aforementioned wheel-rail contact force transducer.

It is important to underline that the Flextrack permits track parameters to be defined for different parts of the track (down to a single sleeper) allowing a simulated track with quasi-continuously varying parameters to be defined which continuously interacts with a running vehicle, a feature that can be hardly realized by using closed-form approaches.

Simulations are complete and are presented for a flexible tangent track where in practice only vertical forces are present, while further simulations are currently in progress to complete the evaluation also in the curved track case and the resulting presence of lateral forces.

Results show that, even for just the tangent track case, a correction on the strain signal produced by the sensors should be made to compensate for the effect of the speed, and that at higher speeds attention should also be paid at the interaction of two subsequent wheels.

Moreover the simulations show that the effect of global track stiffness allows for a certain tolerance to this parameter as long as its variations can be accepted without compromising the functionality of the transducer.

The effects of a single "poorly tamped sleeper" defect were analysed suggesting that the length of track that should be kept in good conditions around the measuring system installation.

It can be concluded that the use of a methodology which combines the use of an advanced multibody simulation and a detailed finite element analysis is a powerful tool to assess the robustness of any wheel-rail force measuring system, including the one proposed and installed by the authors.

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