M26 motorway the M26 at that time formed part of Ringway 4 and would have been designated as part of the M25. Construction of the first part of the M25 began in 1972 but before it opened, plans for the "London Orbital" motorway were modified to combine the southern and western part of Ringway 4 with the northern and eastern part of Ringway 3. To connect the two separate routes, which together were numbered as the M25, an additional section of road needed to be planned and the M25 route was diverted northwards from junction 5 to meet Ringway 3 at Swanley
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