The answer depends in large part on the quality of coal being burned. Good quality anthracite burned clean, so it didn't leave much residue. Some of the sub-bitumenous coals that railroads could buy cheap had lots of non-burnable inclusions that could leave a wheelbarrow load in a smokebox after a comparatively short run.
Early on, the smokebox door would be opened for inspection and cleaning at the end of each run or shift. Later the runs got longer, and some locomotives were fitted with ash hoppers (with doors that allowed emptying) at the bottom of the smokebox.
One event that would lead to an inspection at the earliest convenient moment would be having part of the firebed sucked through the tubes into the smokebox. Wheel slip with minimum cutoff could do it. Then there would be an active fire right under the netting, and it wasn't designed for that.
Chuck
How often does the smokebox and exhaust netting on a coal burning locomotive need to be cleaned and/or inspected??? I assume that small grains of unburned coal and cinders (small bits of coal ash) accumulate in the bottom of the smokebox as the engine is worked, but don't have feel for how often the interior of the smokebox needs to be accessed.
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