A train from Aberdeen (with first class at the front heading south) would reverse at Edinburgh to head towards Carlisle, putting first class at the back.
Train would thus arrive at Newcastle from Carlisle with First Class still at the back, so would need to reverse again at Newcastle to keep first class at the south end.
Altetnatively, First Class can be kept at the front throughout by looping via the Edinburgh South Suburban, and then again via the High Level Bridge at Newcastle, without any reversals.
Suspect train crew would be changed anyway at both Edinburgh and Newcastle, which would speed any necessary reversal anyway.
I'm expanding slightly on what others have mentioned above.
In normal circumstances, First Class is at the front of the train facing Berwick upon Tweed (the start and end point of the North British Railway). By leaving Waverley along the normal route, as most of us know, First Class would be at the buffers upon arrival at London KX.
If departing Waverley heading in the direction of Carlisle (the start and end point of the Caledonian Railway), First Class is at the rear for the Waverley - Newcastle Central leg.
As the Newcastle & Carlisle Railway runs on the south bank of the mighty Tyne between Gateshead Metrocentre and NCL, it approaches NCL from the south. It can either enter NCL via the King Edward Bridge or the High Level Bridge. However, when departing NCL to continue south to KGX, it has to go over the same bridge that it entered the station by so as now to keep First Class facing London.
If the train departed Waverley towards the Berwick direction and was routed via the South Surburban, First Class would be at the front on entering NCL. In this instance, the train would have to depart from NCL towards KGX via the opposite bridge to which it had entered )e.g. enter via King Edward, depart by High Level) so as to have First Class facing London after crossing the mighty Tyne again.
Hope that clarifies a few things for you.