I had never seen this before, first thought was a dirty fire during warm up …

Second thought is: there is an ash pit there and extra cinders fly during the shake down of the grate, plus fires have been known to be lifted out of the box by strong drafts. Best to dump it on ashes.

You can see the ash cart dump track, just above the exhaust pipe.

A method of cleaning out the accumulation of cinders out of the smoke box ? Looks like on that locomotive getting into it to clean would be difficult.

What the heck …? It brings to mind some things I’ve seen in 1950s April issues of Model Railroader magazine.

I found this - A familiar story for Classic Train Forum readers. Do a find for “Miningman” and it’s in the 11th occurrence, sounds reasonable.