In the early 1970s, the 1938 Tube stock on the Bakerloo and Northern line was life-expired and due for replacement. Tentative designs for a new Northern line fleet were abandoned when the go-ahead was given for the Piccadilly line to be extended to Heathrow Airport.
[img] https://upload.wikimedia.org/wikipedia/commons/thumb/8/8f/1972Mk2-OriginalLivery-SouthKenton.jpg/2560px-1972Mk2-OriginalLivery-SouthKenton.jpg[/img]