People said William and Middleton visited Newbridge ambulance response station, where they spoke with first responders about the challenges of working through the pandemic.
William and Middleton will make 10 stops in England, Scotland and Wales over the next three days, according to Today. The stops are being kept secret and social distancing measures will be enforced.
During the tour, William and Middleton will also highlight the work done by NHS Charities Together and the arts, heritage and live performance sector.
William and Middleton are traveling by the royal train, which is mainly used by William's grandmother, Queen Elizabeth II, and father, Prince Charles. William and Middleton left a note thanking transport workers before departing Euston station in London Sunday evening.
"Thank you to transport workers everywhere for keeping the country moving throughout this difficult year. Wishing you all a very Merry Christmas!" the message reads.
A rep for William and Middleton said the tour is meant to thank workers and spread holiday cheer.
"The Duke and Duchess are very much looking forward to shining a spotlight on the incredible work that has been done across the country throughout this difficult year and to sharing their gratitude on behalf of the nation for all those supporting their local communities ahead of the Christmas holidays," the rep said.