The tram that once ran underground through Central London and you can still spot the entrance - MyLondon
... An extension was later added to the line, with through services
starting in 1908 and running from Highbury, further north than Angel,
reaching Holborn, then branching out either south to Kennington or east
to Tower Bridge.
It was quickly discovered that the Kennington
service was not commercially viable, and so services were extended to
Queens Road in Battersea instead, although the low bridge meant only single decker trams could run on that part of the line.
While
it may seem strange to think about these now, double decker trams were
brought in during the 1920s as a way of the subway remaining profitable.
As
the years went on, routes were changed and extended across the tramway
network. By 1931, the routes were Hackney to Wandsworth or Tooting,
Highbury to Waterloo or Norbury, Archway to Kennington and Leyton to
Westminster. ...