What is the normal position of the retainer valve during operation?

The retainer valve is typically positioned in the EX (exhaust) position during operation. This position allows for the proper flow of air in the braking system by enabling the release of excess air and ensuring that the system maintains the necessary pressure balance. The EX position is crucial for the effective function of air brakes, as it prepares the brakes for application and release by preventing any unintended pressure buildup that could result in brake drag or other operational issues.

While the other positions may refer to varying states in which the retainer valve can exist, they do not represent the normal operational state. The importance of the EX position lies in its role in maintaining safety and efficiency in train braking systems.