Elstree South tube station (usually just Elstree on Underground maps) was an unbuilt London Underground station in Elstree, Hertfordshire. It was designed by Charles Holden. The planned location of the station was adjacent to where junction 4 of the M1 motorway was subsequently built.
The extension was part of the Northern Heights project which was intended to electrify a number of steam-operated London and North Eastern Railway (LNER) branch lines and to incorporate them into the Northern Line. The powers to build the extension came from the purchase in 1922 of the unbuilt Watford and Edgware Railway which had planned an extension of the Edgware, Highgate and London Railway to Watford Junction via Bushey, but had never been able to raise the capital required for construction to start.
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 Bushey Heath had taken place. The route of the new rail line had been laid out and some earthworks had been constructed. Adjacent to the site of Elstree South station a pair of 500m tunnels were constructed through a hill. The northern tunnel mouths would have opened immediately before the station.
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 legistation created the 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 cuts through the tunnel alignment. The route of the track bed to Bushey Heath was later followed by the M1 motorway and A41 as far as the A41's junction with the A411.