Why do the brakes NOT apply in emergency when cars are properly uncoupled?

When cars are properly uncoupled, the reason the brakes do not apply in an emergency is primarily due to the configuration of the braking system, particularly the brake pipe. When the cars are uncoupled, the brake pipe is closed at both ends, which means there is no airflow through the system to signal the brakes to apply. The emergency brake mechanism relies on a reduction in air pressure within the brake pipe; if the connection is interrupted or closed, the necessary pressure change to trigger the brake application cannot occur.

In contrast, other options may introduce different scenarios but are not applicable in the case of properly uncoupled cars. For instance, low reservoir pressure does not directly prevent the brakes from applying, as sufficient pressure in adjacent connected cars can still allow for brake application. Activating the emergency brake would typically lead to a brake application rather than preventing it. Similarly, if dynamic brakes are engaged, that pertains to a different aspect of braking and does not prevent the emergency brake from functioning as intended.

Thus, the functioning dynamic of the brake pipe being sealed on both cars is crucial for understanding why the brakes fail to activate in emergency scenarios when uncoupling has occurred correctly.