The Queen has recently been suffering symptoms of gastroenteritis and this is her first public engagement in more than a week, so the Tube should be quite honoured. Although unsurprisingly the Queen isn't an anorak and doesn't make a habit of visiting stations or travelling on the Tube. She is pictured above on the opening of the Victoria Line in 1969. As you can see she's making the major faux pas of having her bag take up a seat. Still it was only her second trip ever on the Tube.
She last visited a Tube station for a much more sombre occasion in February 2010. She went to Aldgate station to view a memorial plaque dedicated to the seven people killed at Aldgate during the July 7, 2005 bombings.
The Queen, Philip and Kate will view a restored 1892 carriage and then meet station staff and apprentices involved in the restoration of the rolling stock. They won't be travelling on a moving train, but will walk through a new S7 train before the unveils a plaque, naming the train Queen Elizabeth II.
TfL would be missing a trick if they didn't give Kate a "Baby on board" badge - but we'll have to see.
The royal party will also meet staff from the Railway Children charity, an international organisation which works to help homeless youngsters. More on the BBC.
The Queen first travelled on the Tube in May 1939, when she was 13 years old and still a Princess. She was pictured with her governess Marion Crawford and Princess Margaret.
UPDATE - Pleased to see that TfL did give Kate a Baby on Board Badge