Palum
what Suineg set it to
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Not quite so fast.
Steam engines exhaust their combustion gasses through the smoke stack. That's why it's called that, used steam is used for many purposes and is not simply through the stack (superheater, compound cylinders, whistle, blow down valves, etc.). If the fireman is properly tending the fire in a modern steam locomotive, the exhaust steam used to create a firebox draft (which pulls any smoke through the boiler flues) will be the primary visible gas and since it is wet and condensing at this point is white.
However, a few problems with this generalization. First, smoke is always present as a byproduct of combustion, even if it can be consumed by the fire itself in ideal conditions.
Secondly, older engines without stokers and better firebox management tools are much harder too keep clean burning or burn hot enough all the time. Wheel slip, grade changes, etc. can pull too much draft which causes the firebox to not be able to complete combustion and pulls the heavy smoke out. Similarly, too little draft causes smoke to build and choke the fire which can cause black plumes. Of course this condition is common when first firing an engine which is why you will have heavy black smoke when only using the auxiliary draft fans because there's no steam pressure yet.
Third, equipment and firebox design make this vary a lot. Things like a feed water heater reduce temperature variance in parts of the firebox and promotes dryer steam which overall comes across as a cleaner exhaust.
Fourth, there are a lot of people who do or don't associate thick dark smoke with steam engines because practices differed. In England, heavy emphasis was placed on clean burning because of urbanized areas. In the US, many roads did not or could not care about that (only so much heat from a firebox, and lots of heavy trains and steep grades). It wasn't really until the late 1920s with the invention of Lima's 2 axle trailing truck that sufficiently large (and heavy) fireboxes were possible to meet all road locomotive demands. The net effect is many restored engines make photo opportunities and purposefully "make smoke", but also why when you see famous older pictures of engines being worked hard they have black smoke.
In the end, these things mean that as you reduce crew skill or increase the age of the equipment (esp old wood burners) you are likely to see significant black smoke. On well maintained modern steam engines you will see very little, most of the time, especially those tiny European engines.
Solo stoves/smokeless fire pits are a simple example of these concepts and why it's a bit more complex than saying there is or isn't.
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