A chain gang is a form of training used by racing cyclists to replicate race conditions while training. Each rider takes a turn at the front and thus keeping the speed high (higher than you would ride on your own). I find it very good training as it keeps me on my threshold (hard to very hard) for most of the time riding.
We usually meet at CSS at about 7.15pm going down the A23 to Gatwick and turning round. It is fairly fluid, punctuated with stops for traffic lights and even cars sometimes