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.


As other have pointed out, the main utility is to allow easier cleaning of the tank cars from  sediment (solid precipitates).  Usually, there is a drain in the lowest point.
The reason that it is in the middle is that in this way, you can allow double the incline compared to if you had the drain at one of the ends for the same height. For example, if the allowed height difference is about 15 cm, and about 15 m length, then if you had the drain in:

the edge, the angle would be $\arctan\left(\frac{0.15}{15}\right)=0.57^\circ$ or 1%.
the middle , the angle would be $\arctan\left(\frac{0.15}{7.5}\right)=1.14^\circ$ or 2%.

The steeper angle helps gravity assisted cleaning.
