I got the Carry Freedom City trailer as a companion to my Brompton.
One of its merits is with gear loaded in the sling bag in the trailer frame,
you can load the folded bike up, on top of the trailer.. then use the handle
and tow it at a dead run down a long train platform, climb stairs , and so forth.
then once the sling bag is un snapped and shouldered
you can fold the trailer wheels into the frame..
Clever Scots..