Keywords: longitudinal dynamics, wheel rail contact, track switch, admissible longitudinal compressive force, train derailment, multibody train dynamics.

The paper presents the results of a coupled longitudinal-lateral model for train dynamics. A previously developed vehicle multibody model and a three dimensional contact model for adjacent multibody vehicles have been incorporated into the TrainDy software for longitudinal train dynamics, where a simplified interface between one- and three-dimensional (multibody) vehicles has been implemented. The vehicle multibody model is fully three-dimensional, with a three dimensional and elastic wheel-rail contact model, whereas the contact model for adjacent vehicles is partially analytical. This complex dynamic model of the whole train has been employed to perform pushing tests and emergency braking on a freight train, and focusing the attention on some vehicles simulated using the multibody approach. In the tests presented, a series of two axle vehicles, Gbs 254, are modelled by means of a multibody approach. Pushing tests allow the admissible longitudinal compressive force (LCF) to be established for a specified test vehicle, i.e. the LCF that leads the vehicle to reach a prescribed derailment criterion limit. Such admissible values have been computed by means of an automatic routine that changes the level of LCF imposed on the testing vehicle by using both bisection and Newton-Raphson algorithms. Of course, this algorithm runs on trains with the desired number of multibody vehicles and it can compute the admissible LCF considering several test parameters. In this paper, the curvature radius of the S shaped curve is chosen as testing parameter. The second type of tests presented here are based on a more common train operation, the emergency braking, with the aim of investigating the effectiveness of the pushing test procedure to evaluate the admissible LCF. Several emergency braking tests, having the maximum LCF equal to the admissible value for a specific curvature radius, are performed on an S shaped curve from different starting points, in order to compute the actual levels of the derailment criteria. The reported results on the pushing tests show that the admissible LCF are affected by the derailment criterion employed and that the pushing tests are much more severe, with respect to safety, than the emergency braking. As a consequence, the admissible LCF computed by means of pushing tests guarantees a safe operation if the actual parameters match those of the tests, which, unfortunately, is not always the case.

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