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These are OSI stations, despite the distance between them: https://en.wikipedia.org/wiki/Oyster_card#Out_of_Station_Int...

And that's not even the highest distance between two OSI stations. The two furthest apart are apparently Marylebone and Paddington (0.8 mi).

http://www.oyster-rail.org.uk/osi-list/

Not to mention I spent five frustrated minutes looking for the Charing Cross branch of the damn black line, only to realize later that it's coloured green.

Yup, they must be about a mile apart.

0.4 miles.

The point is this: you can touch out and touch in at the other and TfL counts it as the same continued journey (so doesn't charge you for a second journey).