This is locomotive 353 at the Western Minnesota Steam thresher's reunion pulling into the station. I am just about to start my shift as Fireperson and took a few images as it pulled in. The smoke coming out the stack is very dark at this point because the steepest hill on the track is just before you arrive at the station and you cannot open the firebox door to add coal because the cold air hitting the hot parts inside causes warping and lots of problems like leaks. So when we get to the top of the grade and the engine is not working so hard and pulling colder air in we can begin to put coal in because it uses a lot as it is working up the hill as well. What you are seeing is the small particles blowing out the stack from the fresh coal which will fall back to the ground and not remain in the air. That's what gets your clothes dirty when you ride the train. :-)
I have also started another B&W project and this is the first photo in it. If you care to see that version it is at the link in the About Me on the right of the Blog page.