I to am with Russ on this one. I have been fortunate enough to fire oil, coal and wood. Like Russ says, once you have the skill on a coal burner and figure out when and where to place the coal, it is not as physically challenging as you might think.
Those "grease burners" are always mentally challenging and you have to be paying attention to everything all the time, up hill, down hill.....all the time.
The wood burner that I have been able to spend quality time on is the Eureka, When working hard it will consume wood pretty rapidly. It doesn't make much difference where you throw it in the firebox...just get it in there and keep doing it at a steady pace.
Various locomotives and railroad profiles make things different for all three. The quality of fuel (oil, coal or wood), the railroad, the engine and the engineer are all variables.
MD Ramsey