Great shots!

That is mostly smoke, the cylinders, whistle and pressure release/safety valve are putting out pure white steam. Though the used steam from the cylinders is routed through the stack to increase draft and clean the inside of the smoke stack of soot and kill sparks. Sparks were more of a problem with wood burners than coal and even less with oil burners but they all have soot to clean out of the stack. When the firebox is good and hot and the fuel air mixture is balanced the smoke gets lighter and especially with oil burners can be almost clear.
The bursts of steam cause the smoke to go straight up for a bit but as the steam and unburned fuel particles dissipate the smoke can change color and starts streaming back instead of up. Since it is in bursts it makes the smoke "puff" in rhythm with the sound of the "chuffing" of the cylinders.

Very nice photos!!! My best friend and I went on a motorcycle trip to WV and one of our "to-do's" was Cass Railroad. I loved riding on one of those Shay trains. It was the highlight of the trip for me. My grandfather on my dad's side was an engineer. He died before I was born so I didn't get to hear any of his stories. I do think that there are some traits that are genetically passed down. I get chills and goosebumps every time I hear a steam whistle. My mom's dad loved photography and motorcycles. I found that out after he passed. I guess you can say that I definitely follow in their hobby footsteps.

Thanks for sharing!!!

Dodie