009 bogie flat wagon

'Ladies and Gentlemen, in the blue corner the very exciting white tiles with anti-mould grout, and in the red corner the rescue flat wagon. A cleanish fight, not too much grout on the dressing gown, and despite the lack of super glue the rescue flat wins by a hair!'

...there  still being12 boxes of tiles in the kitchen, and here is the flat perched on TC with couplings stuck on with UHU. Despite it's length, being based on a Sandy River drawing from David Evans, it rolls round TC without hitting anything.
Any reason why this batch of Greenwich couplings has flaky blackening?