Steamy

Most of what's coming from the smokestack is actually steam, although mixed w/ smoke from the burning coal. Just observe how white it normally is, and how fast the "smoke" dissipates and disappears behind the locomotive. Pure smoke won't do that. Moreover, when the engine pistons are fully decompressed (the phase before new compressed steam flows in), the "used" steam from the last stroke must be let out. Just observe how the steam pulsates through the smokestack in sync with the beat from the working engine. The smoke from the furnace alone won't do that either. The used engine steam therefore flows up the smokestack, to prevent it escaping down by the pistons/wheels, which would hamper the sight from the cabin. Hence, in my view, any steam engine is qualified for this challenge. No?