Docklands Light Railway (DLR)
DLR is a light rail service that serves, er, Docklands in the east. It visits Canary Wharf, Stratford and goes south of the river to Greenwich and Lewisham.
It connects with the Underground in many places, including Bank and Stratford. It meets trains at Stratford, Limehouse and Lewisham and other places.
Almost all the service is above ground. Stations are mostly open with the usual standalone Oyster readers. Touch in and touch out.
Not all trains go to all destinations so you may need to change. Poplar and Canning Town are popular interchange stations.
Sit in the front and pretend to be the driver.
Apparently the Elizabeth line is the UK’s busiest. I assume that’s passengers.