The Queen is making a surprise appearance at Paddington Station to see the completed Elizabeth line, which is named in her honour.
The 96-year-old monarch, who now rarely carries out public engagements outside of her royal residences, was joining her youngest son the Earl of Wessex for the official visit on Tuesday.
Her attendance was not publicly announced in advance, with the head of state facing ongoing mobility problems, but organisers were told there was a possibility she might be able to attend.
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 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.
They were meeting staff who have been key to the project and who will run the railway, including train drivers, station staff and apprentices.
The Elizabeth line, named in honour of the Queen, will open to passengers on May 24.
Crossrail, the project to build the new east-west railway, was delayed and over budget due to numerous issues including construction difficulties and complications installing signalling systems.
It was due to be completed in December 2018 and was set a budget of £14.8 billion in 2010.
The total cost has been estimated at £18.9 billion, including £5.1 billion from the Government.
The Elizabeth line will boost capacity and cut journey times for travel across the capital.
It will stretch from Reading, in Berkshire, and Heathrow Airport, in west London, to Shenfield, in Essex, and Abbey Wood, in south-east London.
Trains will initially operate in three sections, which are expected to be integrated in the autumn.