What term refers to the movement of part of the coupled train at a speed different from another part of the train?

The term that refers to the movement of part of the coupled train at a speed different from another part of the train is slack action. This phenomenon occurs when there is a difference in speed or propulsion between sections of a train, causing the couplings to tighten and slacken as different parts of the train respond to the braking and acceleration forces inconsistently. Slack action can affect the handling of the train, as it creates variations in force along different sections, which can lead to issues such as jerking movements or even derailments if not managed properly.

Draft action, on the other hand, relates to the forces that occur as a train pulls a load, whereas coupling action focuses on the connection points between cars. Braking action describes the process by which the train slows down when brakes are applied, but does not encompass the concept of differential movement between coupled sections. Understanding slack action is crucial for safe and effective train handling, especially in long trains or those with varied cargo.