Field-validated modelling of coupler forces during freight-train start-up on a steep gradient

Restarting a heavy freight train on a steep gradient produces transient tensile forces because locomotive tractive effort is transmitted through sequential coupler-slack closure and nonlinear draft-gear loading. This study develops a longitudinal multi-body model in MSC Adams for freight-train restart on the Angren–Pop mountain railway. The technical contribution is the explicit representation of paired coupler components as finite-mass intermediate bodies, combined with distributed slack, nonlinear loading and unloading branches, damping, preload, time-dependent traction build-up, and local gradient resistance. The baseline numerical case comprises 34 wagons with a total mass of 2800 t, a 27‰ uphill gradient, and 50 mm nominal coupler slack. The maximum calculated tensile force at the investigated connections is 902.13 kN. Full-scale records comprise a principal 36-wagon train of 2852 t, for which the maximum measured force is 931 kN, and an auxiliary 31-wagon train of 2689 t, for which the measured maximum is 863 kN. The baseline simulation is used for mechanism analysis, whereas the spatial model-to-measurement comparison for the principal field consist gives R 2 = 0.9547 across four instrumented positions. The previously used phrase “97% agreement” is removed because it did not define a statistical metric. The results show that gradual traction-force build-up can keep coupler loading below the 1000 kN operational reference adopted in the test programme for the investigated configurations. The conclusions are restricted to the documented train formations, gradient profile, and traction regime.