It may not be apparent at first, but that box on the rear carrier holds some 30 kg of books (66 lbs for the metrically- challenged) acquired by me in the summer of 1994 in Warsaw, Poland, and about to be sent on their way by rail to Stockholm, Sweden. Because of Brompton's low center of gravity, and the necessary space to place and attach the heavy box on the low carrier platform, I was physically able to lead (i.e. not ride) the heavily- ladden bike to the Central Station. I doubt that I'd have it so easy with an upright bike, where the same box on a backrack would have resulted in a much higher center of gravity, ergo much more harder to balance en route.

Total time to acquire, adapt the size of, and strengthen the box to hold the books:  4 hours; pack it again:  2 hours; bring it down to street level, bike and box in a lift not built for simultaneous gravity- assisted transport of bike, box and pilot:  6 minutes; fasten it securely to the bike:  10 minutes; lead it to the railway station:  45 minutes; argue with Railway Staff as to whether it is, or perhaps not, over some arbitrary weight limit:  1 hour; argue with Customs Duty Officer, Sir!, whether newly- acquired books can be classified as unauthorized exports :  1 hour; bike back home SANS the box with a light heart:  15 minutes; fold down the bike for taking it upstairs:  < 20 seconds (I happen to be a pro folder folder/ unfolder).