Looking through old copies of Trains Illustrated, September 1950, I found the following:
The Postal takes the Waverley Route.
On June 8th the Railway Observer records, when the Caledonian main line was blocked at Harthorpe on Beattock Bank owing to the disastrous fire on the 11 am from Birmingham to Glasgow, the southbound "Postal" was diverted from Carstairs to Edinburgh, and came south to Carlisle over the Waverley route, presumably with a double reversal - at Dalry Road Junction and again at Haymarket West junction. The engine was converted Royal Scott No 46145 .
I have not heard of this before. I believe that the Postal left Aberdeen with 4 TPOs; how many did it pick up before Carstairs?
Norman Blackburn