I wonder if this might be related somewhat to the regional being modeled.
The railroads operating in California (the limit of my knowledge <g>)
adopted oil very early due to the lack of coal and oil was very clean
burning. (The photos by Clegg and Beebe of oil burners with huge clouds of
black smoke were staged at the request of the photographers.) Industries in
California also didn't, to my knowledge, rely that heavily on coal. I have 8
color photos of YV freight trains in the early 1940s and about 75-80% of the
box cars are relatively clean....the roads represented include the SP, ATSF,
GN, and NP, all western roads.