Re: Or as they say in London...
The 6km gap is on the Metropolitan Line between Zones 8 and 9. That is a suburban rail service that for political reasons was assigned to TfL rather than to Chiltern Railways, who share tracks with them on other parts of that line. I would exclude that as an outlier when looking at typical tube journeys.
The average across the whole network by the way is 1.31km, on the Circle Line, it is 770m, source: https://www.whatdotheyknow.com/request/distance_between_adjacent_underg
Biting the hand that feeds IT
