My posting on P1 covered just this. Most Goods Vans in the 19C were very similar in construction and use to those more familiar to us from the 1920s. In fact they were little changed until the later 1960s when goods yards started to decline as inner town means of distribution in the face of larger goods lorries (trucks)
The security and marshalling of these wagons was much more stringent than seems to be the case with US freight cars. Part of the guards duties were to inspect vans and wagons for secure doors and sheeting before moving off.
Apart from that London's Goods Yards were busy places and, outside of that, the chances of JtR finding a convenient goods train stuck at a signal just waiting for him to jump aboard is, I feel, unlikely.