The Queen paid an unexpected visit to Paddington Station to see the finished Elizabeth line.
TfL has announced that the long-delayed Crossrail project will open to the public on May 24.
The 96-year-old joined her youngest son, the Earl of Wessex, for the official visit ahead of the opening.
The longest-reigning head of state is just over two weeks away from her Platinum Jubilee weekend, which will mark her 70th year as monarch.
“In a happy development, Her Majesty The Queen is attending today’s event to mark the completion of the Elizabeth line,” a Buckingham Palace spokeswoman said.
“Her Majesty was aware of the engagement and the organisers were informed of the possibility she may attend.”
The Queen rallied to attend the Windsor Horse Show on Friday and was the honoured guest at the equestrian extravaganza A Gallop Through History near Windsor on Sunday, the first major event of the Jubilee celebrations.
However, the Queen’s engagement on Tuesday is her first outside of the Windsor area since attending the Duke of Edinburgh’s memorial service in Westminster Abbey seven weeks ago.
Prime Minister Boris Johnson, Mayor of London Sadiq Khan, and TfL commissioner Andy Byford greeted the Queen and Prince Edward.
They met key members of the Crossrail project as well as those who will operate the railway, such as train drivers, station staff, and apprentices.
The Queen was also given an Oyster card and demonstrated how to use it at a ticket machine.