The Queen made a surprise appearance to officially open the completed Elizabeth line, named in her honour.
She was given an Oyster card and shown how to use it on a ticket machine.
The 96-year-old monarch, who now rarely carries out public engagements outside of her royal residences, joined her youngest son the Earl of Wessex at Paddington Station for the royal visit on Tuesday.
Royal author Robert Jobson appeared on Dan Wootton tonight to give an update on Her Majesty's health.
He told GB News: "The palace have known for some time you have to pace the Queen.
"She would have moved heaven and high water to make sure she got there. for that and I think the way she performed for that was brilliant.
Robert Jobson
GB News
'Don't expect too much of her, it's almost like she's just run a half-marathon.
"The fact is she managed to do a short engagement, she's done it really well and she's clearly on top form but a lot of people rally in situations where they get over tired.
"Personally I think she should be putting her feet up a lot more."
Dressed in sunshine yellow, the Queen arrived at 11.32am, stepping carefully from the transparent lift while holding a walking stick.
The Queen smiled warmly as she met Crossrail and Elizabeth line workers and walked slowly as she made her way around the station concourse.
Elizabeth line customer experience assistant Kofi Duah said he was “thrilled” to present an Oyster card to the Queen and show her how it could be topped up on a machine.
The Queen did not top up the card, which was pre-loaded with £5 of credit.
Mr Duah told the PA news agency: “I gave her an Oyster card and told her she can tap it on the yellow reader.
“I showed her the current balance and how to top up the Oyster.
“She said ‘Where can I use it?’.
“I said ‘You can use it across our line, so from Paddington to Abbey Wood.
“She said ‘Oh nice, splendid’.”
The Queen
PA
Unveiling a plaque stating that she had “officially opened” the Elizabeth line, the monarch spent 10 minutes in the station before departing in a lift, escorted by her son Edward.
The earl returned to the concourse ahead of a return journey on the railway from Paddington to Tottenham Court Road.
The nation’s longest reigning head of state is just over two weeks away from her Platinum Jubilee celebratory weekend.
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’s outfit was a Stewart Parvin double-wool crepe coat with an A-line silk dress in shades of yellow, royal blue and turquoise, and a matching hat by Rachel Trevor-Morgan.
She was wearing her Singapore brooch.
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.
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.
The Queen and Edward were welcomed by Prime Minister Boris Johnson, Mayor of London Sadiq Khan and Transport for London commissioner Andy Byford.
Following the Queen’s visit, Mr Johnson told the invited guests: “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.”