Tom Anderson wrote:

How about making the branch part of the Central Line? You'd need
about a mile of four-tracking to get to the junction from Ealing
Broadway; would that be at all feasible?

Nope. You'd have to knock down most of Ealing Broadway to get enough
space in the GWML corridor, much less the need to rebuild the entire
station to accomodate the extra tracks.

A tunnel would be a possibility, but you'd probably end up missing out
Ealing Broadway itself unless you built tunneled platforms.

IOW, _expensive_....