Yes, it used to be a Newcastle to Stranraer boat train.
The scotrail 156 works to Newcastle to Nunthorpe via the Coast, and return
Hasn't done that for quite a while (at least since December 2019, but I'm sure it was the timetable change in May last year that saw it off). None of the units venture off the Tyne Valley now
The Scotrail units that Northern see through the day are
CK265 1E97 16:13 Glasgow Central - Newcastle (20:07) which then overnights at Heaton (5E97). The next day this forms....
CK277 1N50 06:20 Newcastle to Carlisle which after changing headcode (1L50) goes to Glasgow (not advertised as through train)
The other set is CK272 which is 1N56 07:41 Dumfries to Newcastle which sits at Newcastle from 09:54 until 17:23 before working 2N27 to Carlisle which then forms 1L73 19:20 Carlisle to Glasgow (again not advertised through service)
Could it have been the 0618 from Dumfries?
2N10 06:18 Dumfries to Newcastle is a Northern unit (HT663), another Northern unit works the 07:14 from Dumfries to Newcastle as 2L73 / 2N16 (split headcode at Carlisle)