Trams
In the south of London there is an extensive tram network. It covers Wimbledon in the west towards Croydon in the east.
You can use Oyster or contactless. Use them like a bus, so touch in when you board the tram but not when you get off.
It’s unlikely you will need this, but tram passes are also available.
Apparently the Elizabeth line is the UK’s busiest. I assume that’s passengers.