Thats what you get when braking whilst it is raining, it then drys like that. It is a mixture of brake block dust, accumulated crap off the bogie and other debris off the rail/wheel tread.
The 4s were prone to oil/fuel leaks therefore the centre of the loco often ended up either very oily or a much darker colour with the mix of oil and brake block dust. The oil/fuel/water mix would come from the bedplate drains that were often left open or even when shut could poor from various opening underneath. Oil/fuel mix would often be picked up by the rad fan and blown all over as well, might also be made worse if an oil radiator had split too........
Al Taylor