London Underground ticketing destinations and most interchange stations are in zone 1, meaning that most journeys over similar distances will cost the same. A few stations in the north east of the network, on the Central line, outside Greater London (in the Epping Forest district) are in zones 4, 5 and 6. In the north west of the network, on the Metropolitan line, zones 7, 8 and 9 (formerly A - D) cover stations outside Greater London including Amersham and Chesham in the Chiltern district of Buckinghamshire. Unlike the lower numbered zones, these ancillary areas do not encircle the capital. Some stations are
London–Aylesbury line west, while the Aylesbury line continues north parallel to the underground lines. The line then joins the Metropolitan line tracks a few yards south of Harrow-on-the-Hill station and shares this track with the London Underground's fast Metropolitan line services to Amersham from Baker Street. Beyond Moor Park, the line to Watford diverges. Between Rickmansworth and Chorleywood, the M25 motorway crosses on a viaduct. After Chalfont & Latimer station, the Chesham branch diverges, and the main line continues to Amersham, the terminus for Metropolitan line trains. Beyond Amersham, the line returns to Network Rail control and runs northwest to Aylesbury, following