@mjcarchive I don’t think I explained well enough.
Yes, there is a central turn-back siding at Marbel Arch
and Liverpool Street.
The operational problem is that to turn back at these points means that you have to have the driver check that all the passengers have detained, which can take several minutes (which doesn’t need to happen when the turn-around is in a Spanish-solution place).
So there is a delay to detrain and then another delay as the train then has to wait for a gap in the services running in the opposite direction.
The core of the Central line has trains every two minutes, so it’s totally incompatible to this type of turn around. Also there’s little point as huge numbers of shops/homes/jobs out in Zone 2 (both Westfields for a start) so the is nothing to gain from chucking out Central line passengers in Zone 1, as they would just be trying to cram on to the already-full train behind.