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](https://en.wikipedia.org/wiki/DOT-117_tank_car) 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.

[![drawing of a dot 117 railroad car][1]][1]


  [1]: https://i.sstatic.net/cvhkP.jpg