The Aberdeen to Elgin mail was a continuation of the 0105 Perth to Aberdeen passenger and mail which was itself a continuation of the 2050 Carlisle to Perth. As reflected in Hellfire the 2050 was electric hauled to Mossend and then a 26, 27 or occasionally a 37 would take the train forward.
A leap for this train was quite often made by boarding an Airdrie service at Queen Street low level and leaping at Coatbridge Sunnyside for a walk down to Coatbridge Central via the chip shop (No fish supper and IRN BRU in Hellfire but you may get lucky and a class 20 might drop on one of the electric service drags prompting a quick revision of moves

). The 2050 from Carlisle used to terminate in one of the south facing bays at Perth. The loco then propelled the train out of the station as the 0105 to Aberdeen or occasionally a Perth 08 was used to draw the train out to the south to allow the service to gain the Dundee line.
Those in the know did make use of the run from Aberdeen to Elgin as an alternative to being kicked out on to the mean streets of the granite city at silly hours of the morning. IIRC on reaching Elgin the train was backed into the old station to recess until it was time to carry on to Inverness.
The stock for the 2050 from Carlisle to Perth arrived at Carlisle as a continuation of 1110 or therabouts Perth to Motherwell mail and passenger train in non passenger carrying mode.
Though I do have a few records of having travelled on the train beyond Perth I do not remember much about the formation of the train other than it was a short mark 1 formation with mail and van traffic. It was not unheard of for people to be removed from the train at Aberdeen. I didn't want to run the risk if I could avoid it. My moves tended to revolve around the internal overnights when a class 40 dropped or across the border to Carlisle and back if something interesting was turned out from HA on a working to Carstairs followed by some roarer miles (but that is another story).