|George Dimond, courtesy Central Vermont Railway Historical Society|
Supposedly this made the models look better in the low light levels typical of most model railroads - and all that weathering supposedly made the details "stand out." Really, it was another example of the extreme weathering - to the point of caricature - that gripped the hobby in the 1950s and still hasn't let up in some quarters.
An even better example is the lead image of 4-8-2 no. 600 being readied for another run at the White River engine terminal. In March, 1954 she's not only not gray or "Grimy Black," she's shiny!
The lesson here is to feel free to paint your steamers black, and to take it easy on the weathering.
If you find there's not enough light in the layout room then consider adding more lighting to your layout instead of lightening your locomotives.