In the SMEE cars, what condition prevents emergency brakes from applying when the master controller handle is released?

The correct answer is based on the function of the braking system in SMEE (Subway Multiple Unit Electric) cars. When the master controller handle is released, the emergency brakes are designed to activate unless certain conditions are met.

In this context, if the valve is in full service, it indicates that the brakes are engaged at that level of operation. The system is designed such that if the brakes are actively applying full service, releasing the controller does not trigger the emergency brakes. This ensures that the train can maintain a controlled stop rather than transitioning into an emergency braking situation, which can lead to sudden jolts or instability.

The other conditions listed do impact the operation of the train and its braking system but do not serve the same function in this scenario. For instance, having the reverser handle in a neutral position or the controller set to idle does not prevent emergency brakes from engaging upon the release of the master controller; rather, they might indicate different operational statuses. Low brake pipe pressure could also lead to an inability to engage brakes effectively, but that does not specifically pertain to the circumstance of preventing emergency brakes from applying when the master controller is released. Thus, understanding the specific function of the full-service valve helps clarify why it is the correct choice