An answer appeared in an email from Geoff Gauntlet.
'The women on the coal truck are trimming the load to ensure that nothing can fall off. A lump of coal slipping off the truck as it made its way along the line could do some nasty damage to anyone adjacent to the track e.g. a shunter, permanent way man or even a passenger stood on a station platform. The receiving coal merchant would also be a few lumps short of what was weighted into the wagon at the screens.'
Seems logical to me.
This from and email this morning: