Assessing wagon stability in complex train systems

The long tradition of analysing train dynamics and wagons dynamics separately is supported by both software and standards. The assumption that coupler angles are so small that the consequential vertical and lateral force components can be ignored does not necessarily hold as trains become heavier and longer and coupler forces become larger. Gaps remain in most railways standards. Some possibilities for severe wagon instabilities have already been published, namely wheel unloading due to the lateral components of coupler forces and wagon lift due to mismatches in coupling height. In both these cases, the most severe cases occur when an empty wagon is placed in a loaded train. It is evident that mechanisms for wagon instability can be more complex than in these clear extreme cases. Further, wheel unloading can be added by wagon body and bogie pitch induced by both track irregularity and train dynamics. Just as there has been a tradition of separating train and wagon dynamics, it would also be incorrect to discount the possibility of two or more mechanisms relating to longitudinal dynamics combining unfavourably. A more comprehensive picture of the possible interactions of longitudinal train dynamics and wagon dynamics is presented in this paper, adding the results for wagon body and bogie pitch for homogeneously loaded trains. The paper includes comparisons and combinations of interactions of longitudinal train dynamics and wagon dynamics resulting in some cases to reporting higher results of wheel unloading than previously published. Similar comparisons with track-induced dynamics are also made.