I recently noticed that the cylindrical shape of a railroad tank car is not completely straight but has a bend in the middle. The entire tank is a bit lower there. What's the reason for this bend?

This is also visible on the drawing of a DOT 117 tank car on Wikipedia (the red nearly-horizontal lines were added by myself).

I first thought it had something to do with pressure containment, but the model 117 is apparently used for non-pressured goods only.