In message , Ian Jelf
writes

I was on a late night Piccadilly Line train which stopped at Turnham
Green the other, er, night.

I've often wondered, what's the story behind that oddity? How did it
come to be a feature of the timetable? (I can think of no parallel
elsewhere on the system, at least not in even remotely recent years.)

I think the practice started in the early 60s, when District services
west of London were being cut back. Offering an interchange at Turnham
Green provided greater flexibility, but the additional stop was too
disruptive to the fast-running section of the Piccadilly line to be
offered during the main part of the Monday-Friday timetable.

There was a time in the 70s when the hours of the Turnham Green stoppers
were extended, but they were cut back again due to little usage. In
recent years, there has been much local demand (and a petition) to get
the Piccadilly to stop there throughout the day, but LU has maintained
the position that it would reduce the capacity of the line, at least
until signalling is improved.
--
Paul Terry