Mizter T wrote:

It's also now possible to buy extension tickets from Tube ticket machines
(an possibility which is fairly new I think) - "Extension tickets" is an
option on the main menu screen.

They had something similar back in the 1990s but, in the loads of buttons
era, it was limited to a single zone extension for adults and had a flap
over the relevant button. Otherwise you had to use a ticket office and hope
that the staff would listen and not pull a fast one on you by selling a full
journey ticket.

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