 The Chesham branch is a short single @-@ track railway branch line in Buckinghamshire , England . Although no part of it is within London and it runs entirely above ground , it is owned and operated by the London Underground . It runs from a junction at Chalfont & Latimer with the Metropolitan line and the Chiltern Railways route to Aylesbury , and runs for 3 @.@ 89 miles ( 6 @.@ 26 km ) northwest to its only other station at Chesham . Its terminus at Chesham has since 1961 been the westernmost and since 1994 the northernmost point on the London Underground network . 
