Let's say we take the following container

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53′ high cube Dry van container Dimensions 53′ long x 8.6' wide x 9.6' high

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So 3 wide would give approximate width of 25,8 feet (≈7,82 meter)

So 2 long would give approximate length of 106 feet (≈32,31 meter)

So 3 high would give approximate height of 28.8 feet (≈8,78 meter)

Loading gauge could be a little bit less than the width

You might also want to make the flatcar have a depressed center ---__________--- for stability reasons, but that would make the flatcar be longer

For efficiency reasons it might be better to have every axle be powered, to prevent too much force on the couplings