True, but they require a source of heat. And the question is what to burn or what to use for heating, and in order to be a true steam engine, it should not be e

True, but they require a source of heat. And the question is what to burn or what to use for heating, and in order to be a true steam engine, it should not be electrical or hydrocarbons ( otherwise you don't need an extra engine on top - just burn/use that already). So what's left is carbon in different presentations : wood, or coal, etc.