REPORTER: London's underground railway transports around four million people every day. When it opened on January 9th, 1863, it was the world's first such system and was a welcome relief to the very crowded city, says historian Christian Wolmar.
CHRISTIAN WOLMAR, RAILWAY HISTORIAN: "There were market stalls on the street, there were costermongers, whatever. So the idea came to try to unblock the London streets with an underground railway."
REPORTER: And the pioneering system has just turned 150-years-old. Since its beginning, the Tube has seen numerous additions and improvements. And it's seen destruction … the Underground provided shelter when Germany bombed the city during World War II. More than half a century later, in July 2005, the Tube became a place of terror, when Islamic fundamentalists blew themselves up on three trains. According to Wolmar, the Underground urgently needs new lines to cope with 21st century demands. And love it or hate it, the Tube will have Londoners "minding the gap" for a long time to come.