The idea behind the "change" theory is that the word is often used synonymously for "coins". But we dismissed that theory, basically because KK can be clearly heard and seen on YouTube singing "chains".

Now, those "chains pulled across by platform staff" - that is the sort of explanation I was looking for, if Londoners can positively assure that they existed in the Tube in KK's time. How exactly did they work? Did they replace the doors? Hard to imagine. I remember trams without doors, but underground??