"Tom Anderson" wrote

Aha, i see. But hang on, the way things are now is that as soon as you're
past Turnham Green, there's a pair of tracks that go to Gunnersbury and
nowhere else (and also the two pairs that go to Chiswick Park). The
Central line could surface around there (possibly still being underground
at Turnham Green itself) and use those to get to Richmond, rather than
staying in tunnel to Gunnersbury. That's what i was getting at. Was that
not the situation in 1920?

The junction west of Turnham Green must have been altered in 1932, when the
line from Hammersmith was changed from paired by use (northern pair disused)
to paired by direction. To complicate things, there was a spur from the
Richmond linetowards South Acton (enclosing the 'Gunnersbury Triangle')
which was used for freight (to High Street Kensington) until the 1960s. It
was therefore likely that a convenient location for a tunnel mouth could not
be found until close to Gunnersbury.

Peter