In existing freight-train braking systems, braking is applied on individual cars sequentially at the speed of the air pressure. In the case of long trains that consist of hundreds of rail cars, there is a time delay that results in large and impulsive compressive and tensile coupler forces. These coupler forces compromise the train safe operation and stability, make train handling difficult, cause track damage, raise significantly the maintenance cost, and increase the stopping distances which can lead to serious accidents. The objective of this investigation is to investigate the effect of the brake-delay time on the train longitudinal dynamics (LTD) coupler forces by integrating for the first time detailed three-dimensional coupler and air-brake force models. A spatial non-linear coupler model that takes into account the geometric nonlinearities is employed in order to allow for capturing arbitrary three-dimensional rail-car motion and coupler kinematic degrees of freedom that cannot be captured using existing simpler models. This coupler model is integrated with a detailed air brake model that consists of the locomotive automatic brake valve, air brake pipe, and car control unit (CCU) in order to evaluate conventional air brake force models that account for the air-flow effect in long train pipes as well as the effect of leakage and branch pipe flows. The coupling between the air brake, locomotive automatic brake valve, car control units, and train equations is established and used in the nonlinear LTD simulations, which are performed using the computer software ATTIF (Analysis of Train/Track Interaction Forces). Different LTD braking scenarios are considered and the effect of brake signal time delay on the coupler forces is examined. The results obtained in this study demonstrate the importance of applying all brake forces simultaneously in order to ensure train stability and safety.