Adam Rees Some time ago, (ten years or more) I saw a video on Youtube, entitled "6233 blasts Shap" or something like it. I can no longer find the exact track on Youtube and it may well have succumbed to "link rot".

6233 was the original number of the LMS locomotive "Duchess of Sutherland". The clip was taken by someone leaning out of a carriage window in horrible overcast weather, and showed "Duchess of Sutherland" hauling an enthusiasts' special train (the "Midday Scot"?) up Shap Fell, a notoriously steep gradient on the LMS West Coast main line at a record speed (for a steam locomotive).

Every so often, as the engine was obviously being flogged up the gradient, there was a distinct flash in the column of steam and smoke coming out of the Duchess's chimney. As you suggest, this may well have been unburned coal particles being dragged out of the fire by the powerful exhaust blast. Depending on the grade of coal used, I wonder if there may not also have been some bituminous volatiles from the coal.

The "Duchesses", by the way, were the LMS's and William Stanier's answer to the LNER's and Nigel Gresley's "A4" class which included the record-breaking "Mallard". One of the class, "Duchess of Abercorn", held the record for sustained drawbar horsepower, recording 2500 dhp on Shap and the equally stiff Beattock Bank in Scotland. The LMS was cheating a bit on this occasion, as there were two firemen in the cab. An "A4" briefly exceeded this horsepower, but they may have been cheating also, by "mortgaging the boiler", filling the boiler to the brim with water and piling the fire as high as possible before shutting off the injectors and closing the firehole door. With no cold air entering the firebox and no cold water entering the boiler, the engine could work at over 100% of nominal maximum horsepower for a short but exciting period.

Long discussion over. Suffice it to say that the "A4s" and "Duchesses" working flat out could easily produce a draught in the smokebox which could pull "everything but the grate bars" up the chimney.