I do..
Carry Freedom City is a great companion trailer , for a Brompton.
not only can you tow the trailer behind the bike,
but you can put the folded up bike on top of the trailer
and beat feet to catch the train ..
towing the trailer by hand..
trailer folds flat within its frame, rolls on 12,5" tires,
nylon composite, sealed bearings
sling bag goes in the frame when you are moving..
http://www.carryfreedom.com/city.html#4
http://www.carryfreedom.com/city.html
http://www.biketrailershop.com/ update : sold out at present..
Wiggle in UK might be a source.