Additionally, the locos had received a ton of work in the shop before becoming roadworthy on the D&S. As they didn't change the 461s consist it is apparent that these tests weren't for tonnage purposes, but likely making sure all the related hardware functioned correctly. This was out of the picture when they were worked on, up until now with work on bridge restrictions wrapping up. Forgetting tonnage I would imagine this would be taking a lot less ware off of the single unit.
It's also being pointed out because this marks the first time in history the practice has been done on the D&S, no diesel operations previously had ever been MUed.