Opened in 1863, just a month after the Met line to Farringdon, the world's
second Underground railway was not a passenger line, but a private precursor to
the eventual Mail Rail system. And surprisingly linked to Mail Rail as well.
Worth a read on London Reconnections.

http://www.londonreconnections.com/2015/londons-lost-pneumatic-railway-the-worlds-2nd-oldest-underground/

Nick
--
"The Internet, a sort of ersatz counterfeit of real life"
-- Janet Street-Porter, BBC2, 19th March 1996