This is partly because its southern extension into the outer suburbs was not done by taking over an existing surface line as was generally the case with routes like the Central, Jubilee and Piccadilly lines.
Dette er delvist fordi, den sydlige forlængelse til de ydre forstæder ikke blev foretaget ved at overtage en eksisterende overfladebane, hvilket typisk var tilfældet med ruter som Central, Jubilee og Piccadilly lines.