The safe operation of heavy freight trains on steep mountain railway sections depends on effective control of longitudinal in-train forces during start-up and traction transitions. This study investigates the development of coupler forces during freight-train restart on the Angren–Pop mountain section of Uzbekistan Railways. A longitudinal multi-body model was developed in MSC Adams with explicit consideration of coupler slack, draft-gear characteristics, time-dependent traction-force build-up, track-gradient profile, and the inertial properties of the train consist. The principal numerical case corresponded to a 2800 t freight train starting from rest on a 27‰ uphill gradient with coupler slack of up to 50 mm. The results showed that a gradual increase in traction force by a 2O‘Z-ELR locomotive kept longitudinal forces below the operational safety limit of 1000 kN. To validate the model, full-scale field tests were conducted on the Angren–Pop line. For the principal 36-wagon case with a total mass of 2852 t, the maximum measured coupler force reached 931 kN; a second case involving 31 wagons and 2689 t yielded a maximum of 863 kN. Comparison of the numerical and field results demonstrated good agreement in both the force evolution and the peak values, with an overall model agreement of 97% and a reported coefficient of determination of R² = 0.9734. The proposed approach provides a practical basis for selecting safe restart regimes and assessing coupler loading on steep mountain railway sections.