This was on the BBC news recently. http://www.bbc.com/news/av/uk-england-london-40043215/exploring-london-s-secret-mail-rail
Good story about the underground mail trains in London. No longer in operation, but they were running not that long ago. http://www.atlasobscura.com/articles/tunnels-mail-post-tube-london-rail-railways
re: London's Underground Mail Train