I first saw a model version of this in 2005 at a mini-RPM Bill Welch hosted in northern Virginia. Bill had replacement running boards on several reefer models that were unpainted or a slightly different shade of the roof color. His work influenced my efforts on many models since then. I usually just paint a portion of the running board casting, which is different from Bill’s method of replacing the casting with strip wood. Both methods work and the effect is frequently noted when the models are displayed.
El Paso, TX
This photo link is from the Chicago and North Western Railway Archives Art Collections.
It shows running boards that have been replaced over time. I'm sure it has been noted here previously but this would be a good detail to model.