Which position of the master controller must never be used for running a train?

The position of the master controller that must never be used for running a train is the switching position. This is because the switching position is specifically designed for maneuvering the train during non-operational movements, such as when a train is being repositioned on a yard track or during other switching operations. Using this position while the train is running could lead to a loss of control, as it is not intended for standard operation and could cause the train to operate unpredictably or unsafely.

In contrast, the running position is indeed the proper setting for normal operation, allowing the train to accelerate and decelerate in a controlled manner. The neutral position allows the train to coast without power but is not meant for operational movement while under load. The emergency position is specifically designed for immediate cessation of movement and safety protocols, which are crucial during emergencies but not suitable for regular running. Therefore, the switching position is uniquely inappropriate for running a train, as it represents an operational state that should only be utilized in clearly defined non-operational scenarios.