Final answer:
In the context of subway cars, the designation that does not refer to a subway car coupler is not specified.
Step-by-step explanation:
In the context of subway cars, a coupler refers to the device that connects two cars together.
The three main types of subway car couplers are the mechanical coupler, the electromagnetic coupler, and the AAR coupler.
However, there is no specific designation that does not refer to a subway car coupler, as all designations in this context are related to couplers.