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If a diagonal line (rather than a cross) was painted on one end of the wagon side, I think it was possibly there to denote the position of the vehicles end door if so fitted.
If you mean a white diagonal line it was to show which end had the end discharge door. So staff could see at a glance that all the wagons in a train were the right way round before it started being shunted towards a tippler.
Or in certain cases - ex BR wagons sold off to the then NCB for internal use only in washery plants and coal stocking yards (to stop them creeping out onto the national network)
Quite common in places such as Onllwyn / Gwaun-Cae-Gurwen etc - as it stopped (in theory) main line wagons being used for "internal" reasons.....