Under what condition can hand brakes be released when the engine is coupled to cars on a grade?

The release of hand brakes when the engine is coupled to cars on a grade is appropriate only when the air brake system is fully charged. This is crucial because a fully charged air brake system ensures that the air brakes can be applied effectively to all cars in the consist, providing the necessary control and stopping capability. If the hand brakes were released without a fully charged system, there could be a risk of the train rolling downhill if the grade is steep, leading to potential accidents or runaways. Proper safety protocols dictate that sufficient air pressure is available to maximize braking effectiveness and maintain control over the train during movement.

In this context, the other conditions presented do not guarantee the necessary safety or readiness for releasing hand brakes. Instructions from the train dispatcher or agreement among crew members do not address the technical capacity of the air brake system, while a driver's subjective feeling about readiness does not provide the assurance required for safe operation on a grade.