I am throwing this out as a point of discussion as opposed to making a definitive statement. Take for example the cab forward that is currently being shown on this website’s home page. It has been weathered to a dull gray. I don’t doubt that some steam locos ended up looking like this. Most of the photos we have of steam are in B/W but we do have some color too. My observation is that most of the pictures I’ve seen the locos still look black with some signs of weathering. The same can be said of rolling stock. Of course most freight cars didn’t look like they just came from the paint shop but it seems to me there is a tendency to overdo it. A little rust and a little grime go a long way toward giving a freight car the look of a working piece of equipment. I’m not sure it is necessary to make them look like they just came through a dust storm or were submerged in salt water.
When it comes to weathering, I think there is no one right amount but it seems to me the heavily weathered look is overdone in this hobby.