The route to the north begins at Euston station in London, entering a twin-bore tunnel near the Mornington Street bridge at the station's throat. After continuing through to Old Oak Common underground station, trains proceed through a second tunnel emerging to the surface at its northwestern portal. The line crosses the Colne Valley Regional Park on the Colne Valley Viaduct. The line then enters a tunnel under the Chiltern Hills to emerge near South Heath, northwest of Amersham. It will run roughly parallel to the A413 road and the London to Aylesbury Line, to the west of Wendover. This is a cut-and-cover tunnel under farmland, with soil spread over the final construction in order to reduce the visual impact of the line, reduce noise and allow use of the land above the tunnels for agriculture. After passing west of Aylesbury, the route will run along the corridor of the former Great Central Main Line, joining the alignment north of Quainton Road to travel through rural Buckinghamshire and Oxfordshire up to Mixbury, south of Brackley, from where it will cross the A43 and open countryside through South Northamptonshire and Warwickshire. North of a bored tunnel under Long Itchington Wood, the route will pass through rural areas between Kenilworth and Coventry and cross the A46 to enter the West Midlands.