For several years now they have been building a new railway line across London. It is called the Elizabeth Line, presumably after the queen. It is close to being opened.
The first section will link Paddington and Liverpool Street mainline rail stations. The trains actually go much further although this is probably the bit I would use visiting my brother in Devon.
I am a bit surprised that this does not go to St Pancras (where the Eurostar trains terminate) or Kings Cross.
See https://tfl.gov.uk/travel-information/improvements-and-projects/elizabeth-line .
