AFAIK the chains were used to tip the wagon body. The wagon was stopped (and presumably scotched or spragged) and the loco pulled the chain, thus tiping the body.
Hi Andy. Well done tutorial. I had attempted something similar with sleeper spacing albeit without the spacing tool you mention. Did fine for straight track but what happens on curved track? It was a nightmare and I abandoned the idea. Do you think the spacer tool would still be useful on curves or is there some other technique?