Why are wood burning and coal burning smokestacks on steam locomotive different?

Question

One type is "normal" - same size from bottom to top. The other one is really wide on top. Which is which?

Answer 1

Coal burns hotter, wood needs a spark arrestor.

Answer 2

wood burning needs to supress live sparks, which is the function of the large tapered funnel. the large exit diameter of the funnel reduces the exhaust velocity and hopefully eliminates the embers from being ejected with the smoke stream. generally there will also be a screen cover, not readily seen from the ground.

coal does not have as many embers and the clinkers are generally heavy enough to not go out the stack.