A locomotive is a complex system with numerous subsystems, each subsystem interdependent on other subsystems. An operator aboard a locomotive applies tractive and braking effort to control the speed of the locomotive and its load of railcars to assure proper operation and timely arrival at the desired destination. Speed control must also be exercised to maintain in-train forces within acceptable limits, thereby avoiding excessive coupler forces and the possibility of a train break. To perform this function and comply with prescribed operating speeds that may vary with the train's location on the track, the operator generally must have extensive experience operating the locomotive over the specified terrain with different railcar consists.
Train control can also be exercised by an automatic train control system that determines various train and trip parameters, e.g., the timing and magnitude of tractive and braking applications, to control the train. Alternatively, a train control system advises the operator of preferred train control actions, with the operator exercising train control in accordance with the advised actions or in accordance with his/her independent train control assessments.
The train's coupler slack condition (the distance between two linked couplers and changes in that distance) substantially affects train control. Certain train control actions are permitted if certain slack conditions are present, while other train control actions are undesired since they may lead to train, railcar or coupler damage. If the slack condition of the train (or segments of the train) can be determined, predicted or inferred, proper train control actions can be executed responsive thereto, minimizing damage risks or a train break-up.