Certainly a fair amount of trunk haulage was loco coal - e.g South Wales via Salisbury for the non juiced bits of the Southern Region - ditto Yorkshire to London for the GN and GE. I understand South Wales sheds were under strict orders to load as much coal onto the tenders of outwards trains to London etc as could be safely accommodated to save on "free on board coal haulage" .....difficult to quantify - but not 30%. Some of the coal haulage was for electrification use - block loads Toton - Willesden for Stonebridge Park Power station for the NLL lines , to Neasden for LT - and of course to Durnsford Road SW19...