What must be inspected when freight cars are added to a train that have not been previously inspected?

The correct choice emphasizes the importance of a comprehensive inspection when adding freight cars that have not been previously inspected. This inspection must cover multiple conditions, including checking for any objects that may be dragging below the car.

This thorough approach helps ensure the safety and functionality of the train. Inspecting for items that could interfere with the movement of the train, as well as checking various components such as brakes, wheels, and overall structural integrity, is vital. A focus on just one aspect, such as the brake systems or wheels alone, could overlook critical safety issues that may arise from other factors, such as integrity of the car and any obstructions that could pose hazards during transit.

In this context, overlooking the broader scope of what needs inspection may lead to dangerous situations. Thus, the correct answer reflects the need for a holistic inspection protocol to maintain safety standards for all freight cars being added to a train.