The new Elizabeth Line, also called Crossrail, will open shortly. You can read about it here
This is a new extended underground line, from east to west, and from Chelmsford to Reading. The scale of system is several orders of magnitude bigger than the traditional underground.
The line will also connect, at King’s Cross/St Pancras with HS1 and Eurostar services and with Thames Link overground between Bedford and Brighton.
Basically, London just got four times bigger! That’s a trick.