We used a solid threaded steel rod for the central axle and surrounded this with two hollow metal poles (previously washing line poles). This is so the axle won't bend under pressure. Here is a picture of my Pa cutting the poles to size (again, I'm not safe with the power tools)
Sparks are cool.
Even fuzzy out of focus sparks are cool
Attach Axles and Wheels
After knocking in all the poles into the according holes we attached two locking nuts to the threaded pole to secure the outer poles in place. Then we attached the first hub and fastened that in place with a nut, then the wheel and the second hub. We then secured the whole thing in place with two locking nuts. Wheels!
Repeat adding the wheels x4 then inflate the tires. BAM! One waterslideboxcar... pending finishing touchs.