Construction works on the Northern Heights project began in the late 1930s but were interrupted by the outbreak of the Second World War. Most of the work undertaken to that date had been carried out on the existing LNER branch tracks but some work between Edgware and Elstree had taken place. Land had been purchased along the route of the new rail line and some earthworks had been constructed. North of Brockley Hill station a pair of 500m tunnels were partly constructed through a hill towards Elstree . The northern tunnel mouths would have opened immediately before the station. Also adjacent to the site the carriage depot for the line was completed in 1939 but during the war it was converted to use as a munitions factory. After the war it became Aldenham Bus Overhaul Works until it was closed in the 1980s and demolished in the 1990s.
The sites of all three new stations were in semi-rural locations and, as it had elsewhere, it was intended that the opening of the new section of Underground line would stimulate the construction of new residential estates that the stations would then serve. After the war, new legislation was introduced with the intention of limiting the continuing expansion of urban areas into their surrounding countryside. This legislation created the Metropolitan Green Belt around London and the area designated included the area covered by the new Northern line extension. Without the possibility of constructing the new housing estates, the new line had no purpose and the plans for the extension were cancelled in 1950.
Through disuse the Elstree tunnels became flooded and dangerous. The entrances were filled in and buried in the 1970s and today nothing is visible on the surface. The extensive cutting created for the motorway junction and slip roads crosses the tunnel alignment. Some traces of earthworks including foundations of a viaduct still survive at the site of Brockley Hill station . The route of the line between Edgware and Brockley Hill is now occupied by housing and roads including Sterling Avenue and Shelley Close.