What is the recommended method for testing downward deformation in the gooseneck tunnel plate?

Measuring from the interior top face of the side rail is the recommended method for testing downward deformation in the gooseneck tunnel plate because this point provides a direct measurement that accounts for the structural support and integrity of the rail. This location is critical, as it represents the area where the forces exerted on the gooseneck structure are transmitted. By focusing on the top face of the side rail, you're allowing for the assessment of any sag or deformation that may affect the overall loading capacity and stability of the dry van.

Using other surfaces such as the bottom face or the underside of the flange could lead to misleading results, as they do not represent the deformation impacting the structural integrity of the side rail itself. The top face of the bottom side rail, while closer to the area of interest, does not provide the same alignment as measuring from the interior top face of the side rail. This is why option C is the most accurate and contextually relevant method for evaluation.