Kate was visiting London's historic Baker Street station — known for its link to the fictional detective Sherlock Holmes — for an event marking the 150th anniversary of the city's sprawling subway system, known as the Tube.
She was joined by her grandparents-in-law Prince Philip and Queen Elizabeth II, who was attending her first public engagement in more than a week following her hospitalization for a stomach bug.
The monarch appeared well as she boarded a restored 19th-century train carriage and chatted to subway workers and train drivers about their work. Afterward she unveiled a plaque that named the train after her.
Elizabeth was hospitalized on March 3 after suffering from the symptoms of gastroenteritis.
She was released the next day, but the 86-year-old monarch appears to have taken it easy recently. Officials said Wednesday that two events were canceled last week as she recovered.
Howard Collins, who presented the baby badge to Kate, said the duchess recognized it and asked him how it works. The badges are meant to ease the awkwardness often felt by pregnant women when they ask others to give up their train seat.
"She used to travel on the Tube so she probably saw them then," he told reporters.
Kate is around five months pregnant. She said on Sunday that she doesn't yet know the sex of her first child, but hopes it's a boy.
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