I was wondering about that. Even if the K3 went onto a shed for coaling at some stage of the journey, would the fresh coal have been compatible with a Gresley engine?
You're right, that particular Super D was soon sent back to the LMR after that run, apparently!
In the book
LNWR 8 - coupled Goods Engines there's the anecdote about the Western Region takeover of the Merthyr, Tredegar & Abergavenny section at Nationalisation; until then, excursions to Bournville for the Cadbury factory from that line had been worked throughout by Super Ds, but the new management insisted that ex G.W.
passenger engines should be used from Abergavenny forward in future.
So, on 1st August 1948, the Super Ds on the Pontllanfraith - Bournville excursion came off and were replaced by a 'Hall'. It got off to a poor start - slipped so badly that the train rolled back towards catch points - and altogether lost 2 hours to Bournville.
However, for some reason the Super Ds continued to be used on Barry excursions from the MT&A until closure.
I don't know about beyond there, but they did work at least as far as Marefield North Junction in order to get to Leicester.
Interesting, and I don't doubt you; but they were definitely banned at the Southern end, so all we got at Northampton was 40E WDs, never O!s or O4s (apart from when 64779 somehow sneaked through - it really puzzled everybody when its big smokebox and wheels appeared in the distance) and, in other respects, the Joint Line was, I thought very G.N. North of Melton and L.N.W. South (e.g. signalling, even mileposts.). But I might well be wrong, it was a long time ago it closed, and engine numbers were all that mattered then!