The Queen has made a surprise visit to Paddington Station to see the completed Elizabeth line.
Transport for London (TfL) has announced the delayed Crossrail project will open to the public on 24 May.
Ahead of the opening, the 96-year-old joined her youngest son the Earl of Wessex for the official visit.
The longest reigning head of state is just over two weeks away from her Platinum Jubilee celebratory weekend to celebrate 70 years as monarch.
A Buckingham Palace spokeswoman said: “In a happy development, Her Majesty The Queen is attending today’s event to mark the completion of the Elizabeth line.
“Her Majesty was aware of the engagement and the organisers were informed of the possibility she may attend.”
The Queen rallied to make a trip to the Windsor Horse Show on Friday and on Sunday was the guest of honour at the equestrian extravaganza A Gallop Through History near Windsor, the first major event of the Jubilee festivities.
But Tuesday’s engagement is the Queen’s first one outside of the Windsor area since she attended the Duke of Edinburgh’s memorial service in Westminster Abbey seven weeks ago.
Dressed in sunshine yellow, she arrived just after 11:30 BST stepping carefully from a transparent lift while holding a walking stick.
The Queen and Prince Edward were welcomed by Prime Minister Boris Johnson, Mayor of London Sadiq Khan and TfL commissioner Andy Byford.
They met staff who have been key to the project, formerly known as Crossrail, and those who will run the railway, including train drivers, station staff and apprentices.
The Queen was also given an Oyster card and shown how to use it on a ticket machine.
Unveiling a plaque stating she had “officially opened” the Elizabeth line, named in her honour, the monarch spent 10 minutes in the station before departing, escorted by Prince Edward.
Afterwards, Mr Johnson said: “We’re all incredibly touched and moved and grateful to Her Majesty for coming to open the Elizabeth line today.
“It was fantastic to see her.”
The earl then returned to the concourse ahead of a return journey on the railway from Paddington to Tottenham Court Road.
During the second leg, he was invited to stand in the driver’s cab.
He chatted with driver Carinne Spinola as the train moved, telling her it was “brilliant”.
After stepping off the train at Paddington, the earl added: “That was brilliant. I did enjoy that. It was good fun.”