Due to a shortage of Doncaster Works test coaches 55006 "THE FIFE & FORFAR YEOMANRY" was added to the 1S12 05:50 King's Cross - Aberdeen at Doncaster on the 23rd June 1979 and piloted as far as Darlington, sister locomotive 55012 "CREPELLO" in charge from King's Cross remaining on the train.
55006 was on release from Doncaster following her final classified repair and it was the first instance of Doncaster using a service train in order to test a locomotive 'on release'. Several double headed Deltics followed.