Which London Underground line has the highest number of stations located outside Greater London?

The Metropolitan line has the highest number of London Underground stations located outside Greater London.

Its north-western route reaches into Buckinghamshire, serving locations including Amersham and Chesham. These stations are beyond the Greater London boundary, making the Metropolitan line unusual among Underground routes that extend into surrounding counties.

The line’s geography comes from the historic Metropolitan Railway, whose promoters built outward from London into developing suburbs and rural areas. Its reach helped establish the Metropolitan Railway’s name as a source of the word “metropolitan” in modern transport history.

The Central line also crosses the Greater London boundary toward Essex and Hertfordshire, which makes it the most plausible alternative. However, the Metropolitan line serves the larger number of stations outside Greater London.