Having been given the Round tuit by Graeme, I had no excuse but to get experimenting.
The problem to be solved is that on my tiny indoor layout there are grades as steep as 1:12 or so. I have to keep my wagons as light and free-running as possible to get up the hill, but when descending, they tend to bunch up and then lengthen out again. Note that I use chains as very loose couplings.
So I needed a van with brakes that could easily be manually applied and taken off.
Now my mechanical skills are about zero, but I tried an experiment of forcing a flat piece of styrene sheet onto an axle of a 4-wheel wagon. By making the axles 9mm dia and using some pieces of a rubber band I got enough friction to seem to work. ie it would hold the wagon chains taunt all the way down the hill, but with the wheels still turning.
I knocked up a simple chassis and tried out some ideas.
This video will make it clear (I hope).
https://www.youtube.com/watch?v=yTbsIs9b6NI
Experiments continue........