Tim O’ comments:
I’m with Tim on this – doesn’t take much in the right environment to dirty up a Steam Era Freight Car pretty quickly.
I am reminded of Richard Hendrickson’s article on “Vintage Dating of Freight Cars” or a title to that effect in RMJ or ‘Ding.. That issue is usually nearby, but due to current household projects I have no idea where it is at the moment – I believe it was in the mid to late nineties, but I reserve the right to be wrong, and would welcome a correction on that topic.
In that article, Richard discussed several things, and provided photographic examples of many of them, including (but not limited to) the following:
1) The only pristine car in your fleet is the one that _just_ rolled out of _your_ paint shop. In the steam era, it will have some degree of dirt and grime within hours.
2) New cars built by a builder will be dirty to some degree before they make it to your railroad.
3) Any given lot of cars will have various degrees of weathering – each of those cars has had a unique journey through the elements.
4) The older the lot of cars, the more dirtier the lot will be – again with variations.
5) Reweigh dates also serve as a dating tool.
6) Pay attention to a given railroad’s repainting schedule – some railroads had a ten year cycle, others had different cycles.
7) And even when a given series of cars is scheduled to be repainted, there is no guarantee that any given car will make it home to be repainted.
Bottom line is that just about any given series of cars in the STMFC era will have a wide variety of weathering.
Having said all that, there are untold degrees and variations of weathering. One that comes to mind is ore cars on the iron ranges of Minnesota, Wisconsin, and Northern Michigan. While exposed to the steam and diesel exhaust of their respective periods, these cars, due to captive service, will exhibit a lot of iron ore dust as their primary weathering as compared to equipment operating in other environments. I submit for discussion that only equipment in localized, captive service would have any consistency in weathering across the consist of a given train.