What is the name of the air connection made when coupling a diesel locomotive to freight cars?

The correct answer identifies the brake pipe as the air connection made when coupling a diesel locomotive to freight cars. The brake pipe is a critical component of the air brake system, serving as the main conduit for compressed air that is used to operate the brakes on the entire train. When a locomotive is coupled to freight cars, the brake pipe connection allows the locomotive to control the air pressure throughout the train's brake system.

In this setup, reducing the air pressure in the brake pipe signals the brakes to engage, while re-establishing pressure releases the brakes. This system is integral to ensuring that trains can be safely and effectively stopped from long distances.

Other options, such as the air reservoir, serve different functions—the air reservoir stores compressed air for use when needed but is not directly involved in the connection made during coupling. The control line, which allows for communication between the locomotive and cars, is also important but plays a different role compared to the brake pipe. The service line works similarly to the brake pipe but is specifically used for managing the operation of the braking system in more advanced applications. Thus, identifying the brake pipe as the primary connection reflects a foundational understanding of train braking systems.