Which of the following conditions is necessary for the ME-42 brake valve when not in use?

The necessary condition for the ME-42 brake valve when not in use is the release position. In this state, the brake system is disengaged, allowing for the smooth operation of the train without any brake application, which is essential for safety and functionality. By being in the release position, the valve ensures that air pressure can freely flow through the braking system, preventing unintentional activation of the brakes during routine operation or while the train is stationary.

Being in the release position also prevents potential brake wear and allows for the system to be reset and ready for service when required. This positioning helps maintain the efficiency of the braking system and ensures that the operator has full control when needed. Other positions, such as emergency or service, are not suitable when the valve is not in use, as they would apply the brakes and could impede the train's movement.