The signalling barely works at the best of times. It's literally a museum of technologies, from the pneumatic to the electronic, and it's one of the most common points of failure on the network.

But when it's working it's been improved to the point where there's almost no spare line capacity at peak times.

And if you're making new tunnels it's so difficult and expensive to get a Tunnel Boring Machine into place that it makes no sense to bore short sections.

Crossrail is supposed to be London's east/west express line. Obviously it's not ideal for District Line users, but it should free up some peak hour capacity for Circle and Hammersmith & City journeys which may translate to fewer District/Circle passengers.

There's also talk of a north/south Crossrail 2.0, but that's unlikely to happen for decades.

There are overground express sections on the Piccadilly (District) and Metropolitan (Jubilee) lines but they all keep the lines separate.

There may well not be room for a tunnelled express in the central area. If it were up to me, I'd consider installing a good urban tram link from (say) Earls Court to Embankment.