I like the Chariot (now a Thule brand) hitch .. It uses a ball elastomer , in a socket hitch under the left axle Nut.

Because It being a Ball, when the rear fold is done , it rotates the Hitch around the ball ..
https://www.bikekidshop.com/cart/ima...-stock-2-M.jpg
Minus the QR skewer ...safety strap optional.


I got a Carry Freedom City.. Bike trailer shop in AZ had them, then, but opted out of ordering more when the UK company changed the terms.
https://www.biketrailershop.com/carr...ers-c-198.html
It is a perfect match for Bromptons . it folds flat within it's own frame. (has rear edge rollers to slip it, vertically, between back to back train seats too)
+ because tow bar/handle, comes in at a right angle to the bike, the Ball elastomer hitch rotation works perfectly ..

and you can carry the folded up bike on top of the trailer. Say you have to run the length of the platform to your seat on the train..

In Use Picture:
http://farm1.static.flickr.com/154/4...96e15388_m.jpg

this is another trailer, designed for Brompton travel , a NL company.

https://www.biketrailershop.com/radi...er-p-2683.html