The Elizabeth line
The Elizabeth line is one of London’s newest. It uses some older tracks and a new underground set of tunnels to connect Reading and Heathrow in the west (including Slough, change for Windsor) to Docklands, Woolwich, Abbey Wood and Shenfield in the east. There are many interchanges.
In central London is has stops at Paddington, Bond Street, Tottenham Court Road, Farringdon and Liverpool Street.
For payment, you can use Oyster apart from the bit west of West Drayton or contactless anywhere. Touch in and touch out.
There’s a frequent service. It varies between busy and jam packed. It also has a patchy record for reliability and can be closed at times for works.
It is not an Underground service, just normal trains that happen to run underground for the central part of the journey.
The stations are enormous and the platforms long. Getting down to them can involve long escalators and even longer walks.
Apparently the Elizabeth line is the UK’s busiest. I assume that’s passengers.