Crossrail is a massive, very expensive, 15 year project and it will 'take some strain off the central line in the morning'. Crossrail2 may help relieve the northern and Victoria too, but that's with normal growth rates.

The soil beneath London isn't a infinite resource either, Crossrail 1 was threaded in between other underground services, with a margin of about 30cm in one case I believe.

Mayors seem to have pet transport projects, Ken had buses, Boris had bikes, What's Sadiq's?