I am not sure how I missed these great shots of cargo trikes from beijing on fixed gear fever.
Click pic for source.
Photographer is Daniel Spehr
Note the hugely sagging chain. It was pretty typical of all the cargo bikes I saw in China. My theory is that they come with retrodirect drive and then quickly break that under actual use and convert the chain to single speed normal direct without actually shortening the chain any. Also note the nice head tube truss.