What is the principal safety feature achieved by the coupling upgrade?

The principal safety feature achieved by the coupling upgrade is that stationary cars cannot accidentally roll away if the coupling fails. This significant enhancement in safety ensures that regardless of the circumstances, whether on a slope or during a stationary period, the risk of cars becoming uncoupled and rolling away is minimized.

Modern couplings are designed with features that prevent separation under various conditions, which is critical for maintaining control over the train's composition, especially during loading and unloading at stations. These enhancements also reduce the potential for accidents that could arise from cars moving unexpectedly, thus assuring both the safety of passengers and the integrity of the rail operation.

The other options, while potentially relevant to train performance or operations, do not address the critical safety implications of coupling integrity in the same way that preventing accidental movement of stationary cars does.