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.
A steam train which carried passengers in the 19th Century has returned to mark 150 years since the first tube journey.
A look back at some of the stops on London Underground's 150 year journey.
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.