Tribars + trailer is not a commonly seen combination...
How else are you going to go really fast
That looks awesome, I could fit both my dogs in there! How easy is it to ride uphill with a load in that? I once tried to tow my kids (aged 2 and 5 at the time) in a steel trailer with my BSO and could hardly make it move at all on any incline whatsoever.
I've not tried a serious load, I reckon I had around 15kg of work stuff in it coming home. Climbing was interesting - being smooth really paid off, so sitting and twiddling rather than out of the saddle honking. Being out of the saddle is fine as long as you aren't going so slowly that you are pedaling in squares. It's the momentum that is a challenge - it's effort to get it going, then if you aren't smooth it pushes you forward slightly when you ease off of the dead part of the pedal stroke, then pulls against you as you push down again. I was glad I was on the bike with a triple, giving a 25" granny (though I don't think I ever engaged that, I certainly was approaching it on one particularly stiff climb).
The box is a
Ferplast Atlas 50 professional, which has had a sheet of transparent plastic rivetted to the rear vents (which face forwards on the trailer), to stop draughts. I guess it could be useful when taking animals to the vet, but it is also a useful size for loading with shopping, either putting bags in through the front or removing the lid and using it as a tray.
I recollect that Pete Bird (Swallow Tandems),who at that time lived in the Tannant Valley,carried his labrador in a trailer.The dog was trained to jump out of the trailer when going uphill & jump back into the trailer at the summit 
What he should have done is have a harness for the dog, so it could give him a tow! Many years ago we used to do soemthing similar to that with the doberman we had at the time. When the weather was really bad in winter, we'd go shopping taking a sledge. The dog would pull it home
