The Duke of Cambridge's paternity leave came to an end as he reopened a flagship London station following a billion-pound redevelopment project.
William left his wife and infant son Prince Louis at home to launch London Bridge Station following a five-year rebuild which has created a huge concourse for passengers and other new facilities for the 21st century transport hub.
He was given a present for his growing family - Prince George, Princess Charlotte and the newborn - a book about Southwark Cathedral's cat and laughed when the head of Network Rail joked Louis might one day have a career on the railways.
The duke arrived by train from Blackfriars Station on a new Class 700 Thameslink service, taken from a depot to carry the royal, and was greeted on platform four by Transport Secretary Chris Grayling.
Mark Carne, chief executive of Network Rail which carried out the redevelopment, began by mentioning the birth of Louis on St George's Day - April 23.
He said: "On behalf of everyone on Britain's railways I'd like to congratulate you and the Duchess of Cambridge on the birth of Prince Louis.
"I know that the royal family have always been tremendous supporters and users of the railway and I hope that Prince Louis will perhaps have a flourishing career as a railway engineer.
"I am immensely proud that we brought this wonderful station into service on the very day we said we would, more than five years ago.
"This station has been rebuilt from the Victorian foundations upwards, the team of engineers that achieved this had to carry out this huge project whilst still providing a service for 48 million people who use this station every year."