Amusing story: back when Morro Bay had an operating power plant (that usually ran on gas), one spring, suddenly one week everybody's cars and yards got dusted with a yellow powder. Horrors! Pollutant release from the power plant! They must have switched to high-sulfur oil! PG&E said ... umm, no, no change in operations ... but of course nobody believed them. Actually, it was a common tree in the area that produced a bright yellow pollen and was unusually prolific that year...

Unfortunately, the first response is usually the one that gathers reporters and lawyers, and it's certainly so with steam locomotives. And remember, one reason for replacing steam locomotives in urban area switching was pollution control - small diesels didn't emit as much visible soot (compared to old beat-up hand-coal-fired switch engines). There are so few steamers still around, though, that air pollution should not be an issue for occasional or rural operations. And an "authentic" photo-freight would not be producing a large smoke plume unless the flues are being sanded - Real Railroads would frown on (as has been noted) excessive fuel use.