Hold height constant. Then a Rectangular Prism, with length and width = radius of the cylinder, has greater volume! So why aren't tank wagons (railroad vehicles that transport gas and liquid) Rectangular Prisms?
- A cylindrical structure will have a lower surface-area to volume ratio than any other shape that will fit in the structure gauge of railway infrastructure.
- I can't be certain but that tank looks as though it has been formed by continuously rolling a continuous helical coil from a flat strip. This lends itself to a continuous process of coiling, welding (with the seam passing by the fixed welder) and then chopping the "pipe" into wagon sized lengths.
- The cylinder will be very strong. It appears from the photo that the tank is self-supporting. No need for chassis so there is further weight saving.
- Possibly the weight of the full tank results in close to the maximum axle loading. A bigger tank couldn't be filled (although a shorter wagon could be built).
- The cylindrical shape eliminates all corner stress points and seams except for the end caps.