Which position on the retaining valve must not be used?

The slow direct position on the retaining valve should not be used because it does not allow for the optimal functioning of the air brake system in terms of maintaining proper air pressure. The retaining valve is designed to help keep the air in the brake cylinder during certain conditions, particularly when you may need to release the brakes quickly or if there is a need for dynamic control of the train's braking system. In the slow direct position, the function of the retaining valve is compromised, which can lead to ineffective braking and potentially dangerous situations.

The other positions—high pressure, low pressure, and direct exhaust—are utilized to manage air pressure effectively and ensure reliable brake performance. Each of these positions has a specific function and is designed to either maintain pressure in the brake cylinders or allow for safe and controlled release of pressure. Therefore, avoiding the slow direct position ensures the air brake system operates correctly, enhancing safety and train handling efficiency.