What's next - the Hogwarts Express?
After the royal couple's trip on the Metropolitan Line, Clarence House tweeted this picture of the Duchess of Cornwall at "Platform 9 3/4" at King's Cross station.
Steam train returns for 150th anniversary
A steam train which carried passengers in the 19th Century has returned to mark 150 years since the first tube journey.
Pictures: Stops along the Tube's 150 year journey
A look back at some of the stops on London Underground's 150 year journey.
150 years of the Tube quiz
All day we've been adding fascinating facts about the Tube, which turns 150 years old this week. Now find out how much you know.