JGF, or 'Jolly Good Fun', was a term coined during the period spanning the Deviation and the rebuilding of the line from Dduallt to Blaenau Ffestiniog. The phrase actually came from FR Co.Chairman, John Routly who said that it may have been "Jolly good fun" for volunteers, but many of the staff did not see it that way, working long hours for little pay, and this became symbolic of the attitude of the Company Board towards the staff.
It was also used as a designator for some of the locomotives used during the reconstruction:
- JGF 1 was also known as Jane (Builder's number 8565 of 1940)
- JGF 2 was also known originally as Diana and later The Lady Diana (Builder's number 21579 of 1956)
- JGF 3 was also known as Sandra (builder's number 22119)
- JGF 4 was also known as Sludge (Builder's number 41545 of 1955)
There is some confusion between JGF1 and JGF3 as the names were swapped at one point; the name Cora was given to JGF3 later.