Not sure whether this is pertinant but in Britain the road to driver status used to be via loco cleaner then fireman then driver. Was it like that on the D&RGW? I'd say there were plenty of Otto Perry Photos and other showing somewhat grimy locos.

On that subject what happens first on a freshly painted loco is that once the fire is lit and the coal goes in, brown tar belches out of the stack leaving a dark brown slicky later of tar from the stack downwards on the smokebox. Once its moving that moves back plus the ash starts covering the rest of the loco, progressing as the loco moves on either direction but as it moves mainly forward mosr of the stuff ends up aft of the stack which is why the area foreward of the stack remain closer to its original painted finish.

Mark K
Oxon, England