A bureaucratic paperwork issue, perhaps?
I think the problem is the wheel spacing.
The distance between the outer wheels of the two bogies at the inner ends of the pair might be less than allowed by Cooper Bridge loadings.
This is because all four axles might be on a small bridge or culvert at the same time.
The long drawbars on some paired 100 tonne coal wagons in NSW is to limit such bridge loadings.
This has been important in Queensland for a long time.
When the English Electric 1300 class was designed, they modified the design from the lighter 1620 class.
They wanted to use the same bogie but with a 90 ton locomotive, the wheel spacing used for a 60 ton locomotive was too small.
So EE cut the wooden mould for the bogie frame and inserted a six inch plug to increase the axle spacing.
The cut lines were visible on all the bogie castings.
The QR 2600 class had a much longer bogie wheelbase than the standard GE export locomotive (Compare the NZR Dx) and a similar bogie was used on the WAGR P class (which is used on the lightest WA grain lines.)
The actual axle loadings would depend on the weight of the containers and should not be a problem with empty containers in general.