Queen Elizabeth II is to head below ground Wednesday with Prince Philip and Catherine, Duchess of Cambridge, to mark the 150th anniversary of the London Underground.
The royals are visiting central London's Baker Street station -- a stone's throw from the home of Sir Arthur Conan Doyle's legendary fictional detective Sherlock Holmes at 221B Baker Street.
The queen was forced to cancel some visits last week because she was experiencing symptoms of gastroenteritis.
Once below the surface, the royal party will view a restored underground coach dating back to 1892 and talk to the people involved in bringing it back to its former glory.
They will then walk through a new S7 train, which will be officially named "Queen Elizabeth II" after the monarch. The royals will also meet the head of London Underground and other transport officials.
The London Underground celebrated 150 years in January since the first passengers traveled underground between Paddington and Farringdon on the Metropolitan Railway.
The Tube, as the Underground is known, is a big part of Londoners' life, with more than 1 billion journeys made each year on its network.
The royals will also meet with representatives of the Railway Children charity, which works to help vulnerable children who live alone and at risk on the streets.
Prince William's wife Catherine is expecting their first baby in July.