What is the primary function of the door relay in these cars?

The primary function of the door relay in train cars is to close the control circuit after all doors have been closed. This is a critical safety feature that ensures the train's operation can proceed only when the doors are fully secured, which prevents any accidental opening during travel. By completing the circuit, the relay confirms that the doors are all in the correct position, allowing the train's systems to engage properly and ensuring the safety of passengers on board.

Understanding this function highlights the importance of door relays in maintaining operational integrity and passenger safety. If the relay did not close the circuit when the doors are shut, it could lead to operational confusion and increase the risk of a serious safety incident. Therefore, this function is key to ensuring that train operations are conducted safely and efficiently.