Abstract: During the slope-start process of heavy-haul trains, the air braking force and the downhill sliding force change dynamically, making it challenging to accurately analyze the longitudinal force variations, the magnitude and timing of tractive force application, and the moment of brake and release using theoretical or analytical methods. Current operational practices largely rely on driver experience, lacking robust data support and theoretical foundations, which can easily lead to start failures or excessive longitudinal forces. To address the difficulty of restarting heavy-haul trains on slopes after a stop, this study establishes a simulation analysis model based on air flow theory and multibody dynamics principles. The accuracy of the model is validated by comparing it with field-measured data under conditions of traction, and of brake or of release. Using the Shen12 (12-axle AC electric locomotive) “1+1”C64 train configuration newly operated on the Shenmu–Shuozhou Railway as a case study, the longitudinal dynamic characteristics during the slope-start process are analyzed. Through simulation experiments, the force mechanisms and dynamic responses during the train starting process are revealed, providing optimized operation strategies and proposing rational recommendations. The computational results indicate that the simulation model proposed can determine critical parameters, such as the required tractive force or electric braking force levels and their application timing, for trains parked on long and steep slopes under various air pressure release conditions. This ensures that the train does not slide backward during the starting process while avoiding excessive coupler forces, thereby safeguarding operational safety. The analysis model developed in this study provides a reliable method for simulating instantaneous air brake loads in longitudinal dynamic simulations, improving simulation accuracy and enabling dynamic reproduction of the entire slope-start process. It offers a foundation for optimizing slope-start operation strategies and accident analysis, supporting the development of precise locomotive control and driver-assistance systems.