When Harry Beck released his first diagrammatic map of the London Underground in 1933, a significant portion of the present-day Northern Line had already been constructed. By 1926, the Northern Line had already reached its current southern terminus at Morden and one of its northern termini at Edgware.
The name Northern for the line was officially adopted in 1937, and it was chosen to reflect the extensive network of extensions that were planned as part of the 1935-40 new works programme. These expansion plans were collectively referred to as the “Northern Heights” and were named after the elevated area in the northern part of London that the lines would pass through.
The plan included extending the line several additional miles to the north, utilising the route that had previously been intended for a railway but had not been constructed. During the 1860s, the Watford and Edgware Railway (W&ER) was founded with the intention of constructing a railway connecting Edgware to Watford, extending northward from the GNR line. Despite its ambitions, the project struggled to secure the necessary funds, and as a result, it was unable to proceed. Nevertheless, the W&ER had obtained the right of way for the proposed railway, which subsequently changed hands among various other railway companies until it ultimately came under the ownership of the Underground Electric Railways of London (UERL) in 1922.
In 1924, the UERL had successfully established its own line with a separate station at Edgware. The UERL then formulated plans to construct the extension from Edgware to complete the railway. However, progress on the project encountered obstacles, and no construction had commenced by the time the UERL merged into the London Passenger Transport Board (LPTB) in 1933.
The 1935-40 New Works plan encompassed various projects for the expansion of the London tube network, with a particular focus on what later became known as the Northern Line. The central concept involved establishing three connections to integrate existing railway lines. The first and second schemes would connect Finsbury Park, Alexandra Palace and Highgate.
Finally, at Edgware, the LNER station would be closed, and a new connection would be established into the LPTB station. The line would start at Finchley Central, run through Mill Hill East before Edgware and then be extended a few miles northwards, culminating at Bushey Heath. This extension would include the construction of three new stations, and a large depot at Aldenham (located between the last two stations) would be built to serve the increased number of tube trains required for the expanded line. This work was scheduled for completion in 1941.
The progress of the Northern Line’s expansion was significantly disrupted by the outbreak of the Second World War. However, despite the challenging circumstances, some construction work on the line did continue. In April 1940, tube trains replaced the steam trains operated by the London and North Eastern Railway (LNER) on the High Barnet Branch. Subsequently, in May 1941, the line reached Mill Hill East, effectively completing the construction and establishing the configuration that is recognised as today’s Northern Line.
However, in other areas, all construction activities came to a halt due to the war. The conversion of the remaining section of the Edgware line and the planned extension to Bushey Heath were postponed until after the war concluded. Some initial groundwork had already begun, including the construction of a viaduct near the present-day Spur Road roundabout. This could be seen until a few years ago.