The locomotive became famous in 1925 when the GWR lent it to the London & North Eastern Railway as part of trials against the LNER's then new A1 Pacific Class, a famous example being LNER 4472 "Flying Scotsman". Running from King's Cross to Grantham, and King's Cross to Doncaster, it made the ascent from King's Cross to Finsbury Park regularly in less than six minutes, a feat that the Pacifics were unable to match. "Pendennis Castle" was also shown to be more economical in both coal and water on the test runs, its superiority in burning unfamiliar Yorkshire coal being measured at 3.7lb per mile.