Catherine Unexpectedly Joins The Queen and Prince Phillip On A Tour of Baker Street Station

As rumoured, the Duchess of Cambridge joined the Queen and Prince Philip at Baker Street station today to mark 150 years of the London Underground today.

Over the weekend Hello! Magazine mentioned Catherine had an engagement with the Queen on Thursday, but interestingly, it was never officially confirmed nor pre-announced by the Palace until this morning. It is only the Queen's second public engagement since she fell ill on March 1st with "the symptoms of gastroenteritis" which may have influenced the decision not to announce it.

When Catherine heard the Queen and Prince Philip had planned to undertake the engagement she asked if she could be part of the visit and the Queen formally invited her. A senior royal aide said, "She just fancied it, she asked if she could come along". It is the third time Catherine has joined the Queen on public engagements.

The Duchess was laughingly presented with a brand new accessory - a Baby on Board badge. She was reported to have responded, "I'll make sure I wear it at home." She also admitted she missed traveling on the tube.

" "She said she used the tube on a regular basis before she was married," Howard Collins, chief operating officer of London Underground, tells PEOPLE. 

The badges are worn by pregnant women for fun – and practical reasons. "She asked if it worked effectively, and I said it helps save some people the embarrassment of not noticing," said Collins. "

A close up of the Baby on Board badge.

Catherine wore a dark teal coat by Danish designer By Malene Birger. Laure Laura at Kate Middleton Style found a very similar piece from the 2010 collection. It is thought Catherine's is most likely from the same collection and it's entirely possible she's had it in her wardrobe since then. It's a beautiful coat and the beading adds a fantastic effect.

The Duchess elected to wear her sapphire and diamond earrings.

Her headpiece might be recognized by some as the very same pillbox hat worn to a friend's wedding in 2010.

Catherine carried her black suede clutch and kept her hands warm in her Cornelia James Pure Wool Gloves With Side Bow.

The Duchess completed her look with Episode 'Angel' pumps. She has been wearing them often recently, and sadly they're no longer available.