Which designation does NOT refer to a subway car coupler?

The correct designation that does not refer to a subway car coupler is labeled AC. In train operations, couplers are specially designed to connect different cars in a train or subway system and can be identified by specific codes based on their characteristics or functionality.

The other designations – IB, BC, and DC – are all types of couplers commonly used within subway systems. Each of these refers to a specific type of coupler design that ensures compatibility and safe connection between subway cars, which is essential for train operation and safety.

In contrast, AC does not correspond to a recognized type of coupler in this context, making it the answer that stands out as not fitting within the established categories typically used in subway systems. Understanding these distinctions is crucial for train operators in ensuring they can effectively manage and maintain the connections between cars.